11. Brief Tour of the Standard Library --- Part II

11. 标准库简介 —— 第二部分

11.1. Output Formatting

11.1. 格式化输出

   >>> import reprlib 
   >>> reprlib.repr(set('supercalifragilisticexpialidocious')) 
   "{'a', 'c', 'd', 'e', 'f', 'g', ...}" 
 

   >>> import pprint 
   >>> t = [[[['black', 'cyan'], 'white', ['green', 'red']], [['magenta', 
   ...     'yellow'], 'blue']]] 
   ... 
   >>> pprint.pprint(t, width=30) 
   [[[['black', 'cyan'], 
      'white', 
      ['green', 'red']], 
     [['magenta', 'yellow'], 
      'blue']]] 
 

   >>> 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 strings 
   instead of one big string with newlines 
   to 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. Templating

11.2. 模板

   >>> from string import Template 
   >>> t = Template('${village}folk send $$10 to $cause.') 
   >>> t.substitute(village='Nottingham', cause='the ditch fund') 
   'Nottinghamfolk send $10 to the ditch fund.' 
 

   >>> t = Template('Return the $item to $owner.') 
   >>> d = dict(item='unladen swallow') 
   >>> t.substitute(d) 
   Traceback (most recent call last): 
     ... 
   KeyError: 'owner' 
   >>> t.safe_substitute(d) 
   'Return the unladen swallow to $owner.' 
 

   >>> 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.jpg 
   img_1076.jpg --> Ashley_1.jpg 
   img_1077.jpg --> Ashley_2.jpg 
 

11.3. Working with Binary Data Record Layouts

11.3. 使用二进制数据记录格式

   import struct 
 
   with open('myfile.zip', 'rb') as f: 
       data = f.read() 
 
   start = 0 
   for i in range(3):                      # show the first 3 file headers 
       start += 14 
       fields = struct.unpack('       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. Multi-threading

11.4. 多线程

   import threading, zipfile 
 
   class AsyncZip(threading.Thread): 
       def __init__(self, infile, outfile): 
           threading.Thread.__init__(self) 
           self.infile = infile 
           self.outfile = outfile 
 
       def 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 finish 
   print('Main program waited until background was done.') 
 

11.5. Logging

11.5. 日志

   import logging 
   logging.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') 
 

   WARNING:root:Warning:config file server.conf not found 
   ERROR:root:Error occurred 
   CRITICAL:root:Critical error -- shutting down 
 

11.6. Weak References

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 alive 
   10 
   >>> del a                       # remove the one reference 
   >>> gc.collect()                # run garbage collection right away 
   0 
   >>> d['primary']                # entry was automatically removed 
   Traceback (most recent call last): 
     File "", line 1, in  
       d['primary']                # entry was automatically removed 
     File "C:/python39/lib/weakref.py", line 46, in __getitem__ 
       o = self.data[key]() 
   KeyError: 'primary' 
 
 

11.7. Tools for Working with Lists

11.7. 用于操作列表的工具

   >>> from array import array 
   >>> a = array('H', [4000, 10, 700, 22222]) 
   >>> sum(a) 
   26932 
   >>> a[1:3] 
   array('H', [10, 700]) 
 

   >>> from collections import deque 
   >>> d = deque(["task1", "task2", "task3"]) 
   >>> d.append("task4") 
   >>> print("Handling", d.popleft()) 
   Handling task1 
 
   unsearched = deque([starting_node]) 
   def breadth_first_search(unsearched): 
       node = unsearched.popleft() 
       for m in gen_moves(node): 
           if is_goal(m): 
               return m 
           unsearched.append(m) 
 

   >>> 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')] 
 

   >>> 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. Decimal Floating Point Arithmetic

11.8. 十进制浮点运算

   >>> from decimal import * 
   >>> round(Decimal('0.70') * Decimal('1.05'), 2) 
   Decimal('0.74') 
   >>> round(.70 * 1.05, 2) 
   0.73 
 

   >>> Decimal('1.00') % Decimal('.10') 
   Decimal('0.00') 
   >>> 1.00 % 0.10 
   0.09999999999999995 
 
   >>> sum([Decimal('0.1')]*10) == Decimal('1.0') 
   True 
   >>> sum([0.1]*10) == 1.0 
   False 
 

   >>> getcontext().prec = 36 
   >>> Decimal(1) / Decimal(7) 
   Decimal('0.142857142857142857142857142857142857')