11. 标准库简介 —— 第二部分

第二部分涵盖了专业编程所需要的更高级的模块。这些模块很少用在小脚本中。

11.1. 格式化输出

textwrap 模块能够格式化文本段落，以适应给定的屏幕宽度:

>>> import textwrap>>> doc = """The wrap() method is just like fill() except that it returns... a list of strings instead of one big string with newlines to separate... the wrapped lines."""...>>> print(textwrap.fill(doc, width=40))The wrap() method is just like fill()except that it returns a list of stringsinstead of one big string with newlinesto separate the wrapped lines.

>>> import locale>>> locale.setlocale(locale.LC_ALL, 'English_United States.1252')'English_United States.1252'>>> conv = locale.localeconv()          # get a mapping of conventions>>> x = 1234567.8>>> locale.format("%d", x, grouping=True)'1,234,567'>>> locale.format_string("%s%.*f", (conv['currency_symbol'],...                      conv['frac_digits'], x), grouping=True)'$1,234,567.80'

11.2. 模板

Template 的子类可以自定义定界符。例如，以下是某个照片浏览器的批量重命名功能，采用了百分号作为日期、照片序号和照片格式的占位符:

>>> import time, os.path>>> photofiles = ['img_1074.jpg', 'img_1076.jpg', 'img_1077.jpg']>>> class BatchRename(Template):...     delimiter = '%'>>> fmt = input('Enter rename style (%d-date %n-seqnum %f-format):  ')Enter rename style (%d-date %n-seqnum %f-format):  Ashley_%n%f>>> t = BatchRename(fmt)>>> date = time.strftime('%d%b%y')>>> for i, filename in enumerate(photofiles):...     base, ext = os.path.splitext(filename)...     newname = t.substitute(d=date, n=i, f=ext)...     print('{0} --> {1}'.format(filename, newname))img_1074.jpg --> Ashley_0.jpgimg_1076.jpg --> Ashley_1.jpgimg_1077.jpg --> Ashley_2.jpg

11.3. 使用二进制数据记录格式

struct 模块提供了 pack() 和 unpack() 函数，用于处理不定长度的二进制记录格式。下面的例子展示了在不使用 zipfile 模块的情况下，如何循环遍历一个 ZIP 文件的所有头信息。Pack 代码 "H" 和 "I" 分别代表两字节和四字节无符号整数。" 代表它们是标准尺寸的小尾型字节序:

import structwith open('myfile.zip', 'rb') as f:    data = f.read()start = 0for i in range(3):                      # show the first 3 file headers    start += 14    fields = struct.unpack(', data[start:start+16])    crc32, comp_size, uncomp_size, filenamesize, extra_size = fields    start += 16    filename = data[start:start+filenamesize]    start += filenamesize    extra = data[start:start+extra_size]print(filename, hex(crc32), comp_size, uncomp_size)    start += extra_size + comp_size     # skip to the next header

11.4. 多线程

import threading, zipfileclass AsyncZip(threading.Thread):def __init__(self, infile, outfile):        threading.Thread.__init__(self)self.infile = infileself.outfile = outfiledef run(self):        f = zipfile.ZipFile(self.outfile, 'w', zipfile.ZIP_DEFLATED)        f.write(self.infile)        f.close()print('Finished background zip of:', self.infile)background = AsyncZip('mydata.txt', 'myarchive.zip')background.start()print('The main program continues to run in foreground.')background.join()    # Wait for the background task to finishprint('Main program waited until background was done.')

11.5. 日志记录

logging 模块提供功能齐全且灵活的日志记录系统。在最简单的情况下，日志消息被发送到文件或 sys.stderr

import logginglogging.debug('Debugging information')logging.info('Informational message')logging.warning('Warning:config file %s not found', 'server.conf')logging.error('Error occurred')logging.critical('Critical error -- shutting down')

11.6. 弱引用

>>> import weakref, gc>>> class A:...     def __init__(self, value):...         self.value = value...     def __repr__(self):...         return str(self.value)...>>> a = A(10)                   # create a reference>>> d = weakref.WeakValueDictionary()>>> d['primary'] = a            # does not create a reference>>> d['primary']                # fetch the object if it is still alive10>>> del a                       # remove the one reference>>> gc.collect()                # run garbage collection right away0>>> d['primary']                # entry was automatically removedTraceback (most recent call last):  File "", line 1, in     d['primary']                # entry was automatically removed  File "C:/python38/lib/weakref.py", line 46, in __getitem__    o = self.data[key]()KeyError: 'primary'

11.7. 用于操作列表的工具

在替代的列表实现以外，标准库也提供了其他工具，例如 bisect 模块具有用于操作排序列表的函数:

>>> import bisect>>> scores = [(100, 'perl'), (200, 'tcl'), (400, 'lua'), (500, 'python')]>>> bisect.insort(scores, (300, 'ruby'))>>> scores[(100, 'perl'), (200, 'tcl'), (300, 'ruby'), (400, 'lua'), (500, 'python')]

heapq 模块提供了基于常规列表来实现堆的函数。最小值的条目总是保持在位置零。这对于需要重复访问最小元素而不希望运行完整列表排序的应用来说非常有用:

>>> from heapq import heapify, heappop, heappush>>> data = [1, 3, 5, 7, 9, 2, 4, 6, 8, 0]>>> heapify(data)                      # rearrange the list into heap order>>> heappush(data, -5)                 # add a new entry>>> [heappop(data) for i in range(3)]  # fetch the three smallest entries[-5, 0, 1]

11.8. 十进制浮点运算

阅读笔记

本节简单描述了高阶码农可能会涉足的标准库，看代码一目了然。