395 lines
15 KiB
ReStructuredText
395 lines
15 KiB
ReStructuredText
.. _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, sys
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>>> def raw_input(prompt):
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... sys.stdout.write(prompt)
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... sys.stdout.flush()
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... return sys.stdin.readline()
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...
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>>> photofiles = ['img_1074.jpg', 'img_1076.jpg', 'img_1077.jpg']
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>>> class BatchRename(Template):
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... delimiter = '%'
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>>> fmt = raw_input('Enter rename style (%d-date %n-seqnum %f-format): ')
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Enter rename style (%d-date %n-seqnum %f-format): Ashley_%n%f
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>>> t = BatchRename(fmt)
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>>> date = time.strftime('%d%b%y')
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>>> for i, filename in enumerate(photofiles):
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... base, ext = os.path.splitext(filename)
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... newname = t.substitute(d=date, n=i, f=ext)
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... print('%s --> %s' % (filename, newname))
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img_1074.jpg --> Ashley_0.jpg
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img_1076.jpg --> Ashley_1.jpg
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img_1077.jpg --> Ashley_2.jpg
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Another application for templating is separating program logic from the details
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of multiple output formats. This makes it possible to substitute custom
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templates for XML files, plain text reports, and HTML web reports.
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.. _tut-binary-formats:
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Working with Binary Data Record Layouts
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=======================================
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The :mod:`struct` module provides :func:`pack` and :func:`unpack` functions for
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working with variable length binary record formats. The following example shows
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how to loop through header information in a ZIP file (with pack codes ``"H"``
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and ``"L"`` representing two and four byte unsigned numbers respectively)::
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import struct
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data = open('myfile.zip', 'rb').read()
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start = 0
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for i in range(3): # show the first 3 file headers
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start += 14
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fields = struct.unpack('LLLHH', data[start:start+16])
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crc32, comp_size, uncomp_size, filenamesize, extra_size = fields
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start += 16
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filename = data[start:start+filenamesize]
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start += filenamesize
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extra = data[start:start+extra_size]
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print(filename, hex(crc32), comp_size, uncomp_size)
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start += extra_size + comp_size # skip to the next header
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.. _tut-multi-threading:
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Multi-threading
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===============
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Threading is a technique for decoupling tasks which are not sequentially
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dependent. Threads can be used to improve the responsiveness of applications
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that accept user input while other tasks run in the background. A related use
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case is running I/O in parallel with computations in another thread.
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The following code shows how the high level :mod:`threading` module can run
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tasks in background while the main program continues to run::
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import threading, zipfile
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class AsyncZip(threading.Thread):
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def __init__(self, infile, outfile):
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threading.Thread.__init__(self)
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self.infile = infile
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self.outfile = outfile
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def run(self):
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f = zipfile.ZipFile(self.outfile, 'w', zipfile.ZIP_DEFLATED)
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f.write(self.infile)
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f.close()
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print('Finished background zip of:', self.infile)
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background = AsyncZip('mydata.txt', 'myarchive.zip')
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background.start()
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print('The main program continues to run in foreground.')
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background.join() # Wait for the background task to finish
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print('Main program waited until background was done.')
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The principal challenge of multi-threaded applications is coordinating threads
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that share data or other resources. To that end, the threading module provides
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a number of synchronization primitives including locks, events, condition
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variables, and semaphores.
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While those tools are powerful, minor design errors can result in problems that
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are difficult to reproduce. So, the preferred approach to task coordination is
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to concentrate all access to a resource in a single thread and then use the
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:mod:`Queue` module to feed that thread with requests from other threads.
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Applications using :class:`Queue` objects for inter-thread communication and
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coordination are easier to design, more readable, and more reliable.
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.. _tut-logging:
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Logging
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=======
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The :mod:`logging` module offers a full featured and flexible logging system.
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At its simplest, log messages are sent to a file or to ``sys.stderr``::
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import logging
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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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This produces the following output::
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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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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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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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.. _tut-weak-references:
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Weak References
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===============
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Python does automatic memory management (reference counting for most objects and
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garbage collection to eliminate cycles). The memory is freed shortly after the
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last reference to it has been eliminated.
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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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>>> 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 "<pyshell#108>", line 1, in -toplevel-
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d['primary'] # entry was automatically removed
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File "C:/python30/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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.. _tut-list-tools:
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Tools for Working with Lists
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============================
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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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The :mod:`array` module provides an :class:`array()` object that is like a list
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that stores only homogenous 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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>>> 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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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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middle. These objects are well suited for implementing queues and breadth first
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tree searches::
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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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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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In addition to alternative list implementations, the library also offers other
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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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>>> import bisect
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>>> scores = [(100, 'perl'), (200, 'tcl'), (400, 'lua'), (500, 'python')]
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>>> bisect.insort(scores, (300, 'ruby'))
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>>> scores
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[(100, 'perl'), (200, 'tcl'), (300, 'ruby'), (400, 'lua'), (500, 'python')]
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The :mod:`heapq` module provides functions for implementing heaps based on
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regular lists. The lowest valued entry is always kept at position zero. This
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is useful for applications which repeatedly access the smallest element but do
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not want to run a full list sort::
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>>> from heapq import heapify, heappop, heappush
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>>> data = [1, 3, 5, 7, 9, 2, 4, 6, 8, 0]
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>>> heapify(data) # rearrange the list into heap order
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>>> heappush(data, -5) # add a new entry
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>>> [heappop(data) for i in range(3)] # fetch the three smallest entries
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[-5, 0, 1]
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.. _tut-decimal-fp:
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Decimal Floating Point Arithmetic
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=================================
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The :mod:`decimal` module offers a :class:`Decimal` datatype for decimal
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floating point arithmetic. Compared to the built-in :class:`float`
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implementation of binary floating point, the new class is especially helpful for
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financial applications and other uses which require exact decimal
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representation, control over precision, control over rounding to meet legal or
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regulatory requirements, tracking of significant decimal places, or for
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applications where the user expects the results to match calculations done by
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hand.
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For example, calculating a 5% tax on a 70 cent phone charge gives different
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results in decimal floating point and binary floating point. The difference
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becomes significant if the results are rounded to the nearest cent::
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>>> from decimal import *
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>>> Decimal('0.70') * Decimal('1.05')
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Decimal("0.7350")
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>>> .70 * 1.05
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0.73499999999999999
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The :class:`Decimal` result keeps a trailing zero, automatically inferring four
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place significance from multiplicands with two place significance. Decimal
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reproduces mathematics as done by hand and avoids issues that can arise when
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binary floating point cannot exactly represent decimal quantities.
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Exact representation enables the :class:`Decimal` class to perform modulo
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calculations and equality tests that are unsuitable for binary floating point::
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>>> Decimal('1.00') % Decimal('.10')
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Decimal("0.00")
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>>> 1.00 % 0.10
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0.09999999999999995
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>>> sum([Decimal('0.1')]*10) == Decimal('1.0')
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True
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>>> sum([0.1]*10) == 1.0
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False
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The :mod:`decimal` module provides arithmetic with as much precision as needed::
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>>> getcontext().prec = 36
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>>> Decimal(1) / Decimal(7)
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Decimal("0.142857142857142857142857142857142857")
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