author | Shane McErlean <shane.mcerlean@accenture.com> |
Fri, 27 Aug 2010 15:33:49 +0100 | |
changeset 101 | 1e82a8be084a |
parent 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
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.. _tut-brieftourtwo: |
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********************************************* |
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Brief Tour of the Standard Library -- Part II |
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********************************************* |
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This second tour covers more advanced modules that support professional |
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programming needs. These modules rarely occur in small scripts. |
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.. _tut-output-formatting: |
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Output Formatting |
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================= |
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The :mod:`repr` module provides a version of :func:`repr` customized for |
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abbreviated displays of large or deeply nested containers:: |
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>>> import repr |
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>>> repr.repr(set('supercalifragilisticexpialidocious')) |
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"set(['a', 'c', 'd', 'e', 'f', 'g', ...])" |
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The :mod:`pprint` module offers more sophisticated control over printing both |
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built-in and user defined objects in a way that is readable by the interpreter. |
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When the result is longer than one line, the "pretty printer" adds line breaks |
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and indentation to more clearly reveal data structure:: |
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>>> import pprint |
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>>> t = [[[['black', 'cyan'], 'white', ['green', 'red']], [['magenta', |
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... 'yellow'], 'blue']]] |
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... |
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>>> pprint.pprint(t, width=30) |
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[[[['black', 'cyan'], |
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'white', |
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['green', 'red']], |
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[['magenta', 'yellow'], |
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'blue']]] |
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The :mod:`textwrap` module formats paragraphs of text to fit a given screen |
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width:: |
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>>> import textwrap |
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>>> doc = """The wrap() method is just like fill() except that it returns |
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... a list of strings instead of one big string with newlines to separate |
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... the wrapped lines.""" |
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... |
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>>> print textwrap.fill(doc, width=40) |
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The wrap() method is just like fill() |
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except that it returns a list of strings |
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instead of one big string with newlines |
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to separate the wrapped lines. |
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The :mod:`locale` module accesses a database of culture specific data formats. |
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The grouping attribute of locale's format function provides a direct way of |
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formatting numbers with group separators:: |
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>>> import locale |
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>>> locale.setlocale(locale.LC_ALL, 'English_United States.1252') |
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'English_United States.1252' |
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>>> conv = locale.localeconv() # get a mapping of conventions |
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>>> x = 1234567.8 |
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>>> locale.format("%d", x, grouping=True) |
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'1,234,567' |
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>>> locale.format("%s%.*f", (conv['currency_symbol'], |
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... conv['frac_digits'], x), grouping=True) |
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'$1,234,567.80' |
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.. _tut-templating: |
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Templating |
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========== |
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The :mod:`string` module includes a versatile :class:`Template` class with a |
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simplified syntax suitable for editing by end-users. This allows users to |
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customize their applications without having to alter the application. |
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The format uses placeholder names formed by ``$`` with valid Python identifiers |
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(alphanumeric characters and underscores). Surrounding the placeholder with |
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braces allows it to be followed by more alphanumeric letters with no intervening |
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spaces. Writing ``$$`` creates a single escaped ``$``:: |
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>>> from string import Template |
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>>> t = Template('${village}folk send $$10 to $cause.') |
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>>> t.substitute(village='Nottingham', cause='the ditch fund') |
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'Nottinghamfolk send $10 to the ditch fund.' |
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The :meth:`substitute` method raises a :exc:`KeyError` when a placeholder is not |
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supplied in a dictionary or a keyword argument. For mail-merge style |
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applications, user supplied data may be incomplete and the |
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:meth:`safe_substitute` method may be more appropriate --- it will leave |
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placeholders unchanged if data is missing:: |
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>>> t = Template('Return the $item to $owner.') |
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>>> d = dict(item='unladen swallow') |
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>>> t.substitute(d) |
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Traceback (most recent call last): |
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. . . |
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KeyError: 'owner' |
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>>> t.safe_substitute(d) |
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'Return the unladen swallow to $owner.' |
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Template subclasses can specify a custom delimiter. For example, a batch |
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renaming utility for a photo browser may elect to use percent signs for |
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placeholders such as the current date, image sequence number, or file format:: |
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>>> import time, os.path |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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diff
changeset
|
108 |
>>> photofiles = ['img_1074.jpg', 'img_1076.jpg', 'img_1077.jpg'] |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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changeset
|
109 |
>>> class BatchRename(Template): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
110 |
... delimiter = '%' |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
111 |
>>> fmt = raw_input('Enter rename style (%d-date %n-seqnum %f-format): ') |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
112 |
Enter rename style (%d-date %n-seqnum %f-format): Ashley_%n%f |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
113 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
114 |
>>> t = BatchRename(fmt) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
115 |
>>> date = time.strftime('%d%b%y') |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
116 |
>>> for i, filename in enumerate(photofiles): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
117 |
... base, ext = os.path.splitext(filename) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
118 |
... newname = t.substitute(d=date, n=i, f=ext) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
119 |
... print '{0} --> {1}'.format(filename, newname) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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changeset
|
120 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
121 |
img_1074.jpg --> Ashley_0.jpg |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
122 |
img_1076.jpg --> Ashley_1.jpg |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
123 |
img_1077.jpg --> Ashley_2.jpg |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
124 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
125 |
Another application for templating is separating program logic from the details |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
126 |
of multiple output formats. This makes it possible to substitute custom |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
127 |
templates for XML files, plain text reports, and HTML web reports. |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
128 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
129 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
130 |
.. _tut-binary-formats: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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changeset
|
131 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
132 |
Working with Binary Data Record Layouts |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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|
133 |
======================================= |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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changeset
|
134 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
135 |
The :mod:`struct` module provides :func:`pack` and :func:`unpack` functions for |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
136 |
working with variable length binary record formats. The following example shows |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
137 |
how to loop through header information in a ZIP file without using the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
138 |
:mod:`zipfile` module. Pack codes ``"H"`` and ``"I"`` represent two and four |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
139 |
byte unsigned numbers respectively. The ``"<"`` indicates that they are |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
140 |
standard size and in little-endian byte order:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
141 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
142 |
import struct |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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changeset
|
143 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
144 |
data = open('myfile.zip', 'rb').read() |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
145 |
start = 0 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
146 |
for i in range(3): # show the first 3 file headers |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
147 |
start += 14 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
148 |
fields = struct.unpack('<IIIHH', data[start:start+16]) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
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|
149 |
crc32, comp_size, uncomp_size, filenamesize, extra_size = fields |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
150 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
151 |
start += 16 |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
152 |
filename = data[start:start+filenamesize] |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
153 |
start += filenamesize |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
154 |
extra = data[start:start+extra_size] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
155 |
print filename, hex(crc32), comp_size, uncomp_size |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
156 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
157 |
start += extra_size + comp_size # skip to the next header |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
158 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
159 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
160 |
.. _tut-multi-threading: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
161 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
162 |
Multi-threading |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
163 |
=============== |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
164 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
165 |
Threading is a technique for decoupling tasks which are not sequentially |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
166 |
dependent. Threads can be used to improve the responsiveness of applications |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
167 |
that accept user input while other tasks run in the background. A related use |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
168 |
case is running I/O in parallel with computations in another thread. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
169 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
170 |
The following code shows how the high level :mod:`threading` module can run |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
171 |
tasks in background while the main program continues to run:: |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
172 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
173 |
import threading, zipfile |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
174 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
175 |
class AsyncZip(threading.Thread): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
176 |
def __init__(self, infile, outfile): |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
177 |
threading.Thread.__init__(self) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
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|
178 |
self.infile = infile |
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
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|
179 |
self.outfile = outfile |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
180 |
def run(self): |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
181 |
f = zipfile.ZipFile(self.outfile, 'w', zipfile.ZIP_DEFLATED) |
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Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
182 |
f.write(self.infile) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
183 |
f.close() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
184 |
print 'Finished background zip of: ', self.infile |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
185 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
186 |
background = AsyncZip('mydata.txt', 'myarchive.zip') |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
187 |
background.start() |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
188 |
print 'The main program continues to run in foreground.' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
189 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
190 |
background.join() # Wait for the background task to finish |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
191 |
print 'Main program waited until background was done.' |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
192 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
193 |
The principal challenge of multi-threaded applications is coordinating threads |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
194 |
that share data or other resources. To that end, the threading module provides |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
195 |
a number of synchronization primitives including locks, events, condition |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
196 |
variables, and semaphores. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
197 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
198 |
While those tools are powerful, minor design errors can result in problems that |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
199 |
are difficult to reproduce. So, the preferred approach to task coordination is |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
200 |
to concentrate all access to a resource in a single thread and then use the |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
201 |
:mod:`Queue` module to feed that thread with requests from other threads. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
202 |
Applications using :class:`Queue.Queue` objects for inter-thread communication |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
203 |
and coordination are easier to design, more readable, and more reliable. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
204 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
205 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
206 |
.. _tut-logging: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
207 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
208 |
Logging |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
209 |
======= |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
210 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
211 |
The :mod:`logging` module offers a full featured and flexible logging system. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
212 |
At its simplest, log messages are sent to a file or to ``sys.stderr``:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
213 |
|
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Initial QEMU (symbian-qemu-0.9.1-12) import
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parents:
diff
changeset
|
214 |
import logging |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
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logging.debug('Debugging information') |
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logging.info('Informational message') |
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logging.warning('Warning:config file %s not found', 'server.conf') |
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logging.error('Error occurred') |
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logging.critical('Critical error -- shutting down') |
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|
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This produces the following output:: |
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|
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WARNING:root:Warning:config file server.conf not found |
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ERROR:root:Error occurred |
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CRITICAL:root:Critical error -- shutting down |
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|
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By default, informational and debugging messages are suppressed and the output |
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is sent to standard error. Other output options include routing messages |
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through email, datagrams, sockets, or to an HTTP Server. New filters can select |
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different routing based on message priority: :const:`DEBUG`, :const:`INFO`, |
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:const:`WARNING`, :const:`ERROR`, and :const:`CRITICAL`. |
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|
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The logging system can be configured directly from Python or can be loaded from |
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a user editable configuration file for customized logging without altering the |
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application. |
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|
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|
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.. _tut-weak-references: |
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|
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Weak References |
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=============== |
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|
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Python does automatic memory management (reference counting for most objects and |
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:term:`garbage collection` to eliminate cycles). The memory is freed shortly |
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after the last reference to it has been eliminated. |
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|
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This approach works fine for most applications but occasionally there is a need |
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to track objects only as long as they are being used by something else. |
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Unfortunately, just tracking them creates a reference that makes them permanent. |
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The :mod:`weakref` module provides tools for tracking objects without creating a |
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reference. When the object is no longer needed, it is automatically removed |
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from a weakref table and a callback is triggered for weakref objects. Typical |
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applications include caching objects that are expensive to create:: |
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|
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>>> import weakref, gc |
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>>> class A: |
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... def __init__(self, value): |
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... self.value = value |
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... def __repr__(self): |
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... return str(self.value) |
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... |
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>>> a = A(10) # create a reference |
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>>> d = weakref.WeakValueDictionary() |
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>>> d['primary'] = a # does not create a reference |
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>>> d['primary'] # fetch the object if it is still alive |
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10 |
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>>> del a # remove the one reference |
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>>> gc.collect() # run garbage collection right away |
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0 |
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>>> d['primary'] # entry was automatically removed |
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Traceback (most recent call last): |
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File "<stdin>", line 1, in <module> |
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d['primary'] # entry was automatically removed |
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File "C:/python26/lib/weakref.py", line 46, in __getitem__ |
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o = self.data[key]() |
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KeyError: 'primary' |
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|
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|
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.. _tut-list-tools: |
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|
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Tools for Working with Lists |
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============================ |
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283 |
|
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Many data structure needs can be met with the built-in list type. However, |
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sometimes there is a need for alternative implementations with different |
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performance trade-offs. |
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|
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The :mod:`array` module provides an :class:`array()` object that is like a list |
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that stores only homogeneous data and stores it more compactly. The following |
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example shows an array of numbers stored as two byte unsigned binary numbers |
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(typecode ``"H"``) rather than the usual 16 bytes per entry for regular lists of |
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python int objects:: |
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293 |
|
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>>> from array import array |
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>>> a = array('H', [4000, 10, 700, 22222]) |
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>>> sum(a) |
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26932 |
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>>> a[1:3] |
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array('H', [10, 700]) |
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300 |
|
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The :mod:`collections` module provides a :class:`deque()` object that is like a |
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list with faster appends and pops from the left side but slower lookups in the |
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|
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middle. These objects are well suited for implementing queues and breadth first |
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tree searches:: |
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305 |
|
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>>> from collections import deque |
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>>> d = deque(["task1", "task2", "task3"]) |
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>>> d.append("task4") |
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>>> print "Handling", d.popleft() |
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Handling task1 |
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311 |
|
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unsearched = deque([starting_node]) |
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def breadth_first_search(unsearched): |
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node = unsearched.popleft() |
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for m in gen_moves(node): |
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if is_goal(m): |
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return m |
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unsearched.append(m) |
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319 |
|
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In addition to alternative list implementations, the library also offers other |
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|
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tools such as the :mod:`bisect` module with functions for manipulating sorted |
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lists:: |
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323 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
324 |
>>> import bisect |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
325 |
>>> scores = [(100, 'perl'), (200, 'tcl'), (400, 'lua'), (500, 'python')] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
326 |
>>> bisect.insort(scores, (300, 'ruby')) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
327 |
>>> scores |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
328 |
[(100, 'perl'), (200, 'tcl'), (300, 'ruby'), (400, 'lua'), (500, 'python')] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
329 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
330 |
The :mod:`heapq` module provides functions for implementing heaps based on |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
331 |
regular lists. The lowest valued entry is always kept at position zero. This |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
332 |
is useful for applications which repeatedly access the smallest element but do |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
333 |
not want to run a full list sort:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
334 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
335 |
>>> from heapq import heapify, heappop, heappush |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
336 |
>>> data = [1, 3, 5, 7, 9, 2, 4, 6, 8, 0] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
337 |
>>> heapify(data) # rearrange the list into heap order |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
338 |
>>> heappush(data, -5) # add a new entry |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
339 |
>>> [heappop(data) for i in range(3)] # fetch the three smallest entries |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
340 |
[-5, 0, 1] |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
341 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
342 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
343 |
.. _tut-decimal-fp: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
344 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
345 |
Decimal Floating Point Arithmetic |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
346 |
================================= |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
347 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
348 |
The :mod:`decimal` module offers a :class:`Decimal` datatype for decimal |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
349 |
floating point arithmetic. Compared to the built-in :class:`float` |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
350 |
implementation of binary floating point, the new class is especially helpful for |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
351 |
financial applications and other uses which require exact decimal |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
352 |
representation, control over precision, control over rounding to meet legal or |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
353 |
regulatory requirements, tracking of significant decimal places, or for |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
354 |
applications where the user expects the results to match calculations done by |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
355 |
hand. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
356 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
357 |
For example, calculating a 5% tax on a 70 cent phone charge gives different |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
358 |
results in decimal floating point and binary floating point. The difference |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
359 |
becomes significant if the results are rounded to the nearest cent:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
360 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
361 |
>>> from decimal import * |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
362 |
>>> Decimal('0.70') * Decimal('1.05') |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
363 |
Decimal("0.7350") |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
364 |
>>> .70 * 1.05 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
365 |
0.73499999999999999 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
366 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
367 |
The :class:`Decimal` result keeps a trailing zero, automatically inferring four |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
368 |
place significance from multiplicands with two place significance. Decimal |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
369 |
reproduces mathematics as done by hand and avoids issues that can arise when |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
370 |
binary floating point cannot exactly represent decimal quantities. |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
371 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
372 |
Exact representation enables the :class:`Decimal` class to perform modulo |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
373 |
calculations and equality tests that are unsuitable for binary floating point:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
374 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
375 |
>>> Decimal('1.00') % Decimal('.10') |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
376 |
Decimal("0.00") |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
377 |
>>> 1.00 % 0.10 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
378 |
0.09999999999999995 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
379 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
380 |
>>> sum([Decimal('0.1')]*10) == Decimal('1.0') |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
381 |
True |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
382 |
>>> sum([0.1]*10) == 1.0 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
383 |
False |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
384 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
385 |
The :mod:`decimal` module provides arithmetic with as much precision as needed:: |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
386 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
387 |
>>> getcontext().prec = 36 |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
388 |
>>> Decimal(1) / Decimal(7) |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
389 |
Decimal("0.142857142857142857142857142857142857") |
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
390 |
|
2fb8b9db1c86
Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff
changeset
|
391 |