Python 学习笔记（五）杂项

1. Assert

assert len(unique_characters) <= 10, 'Too many letters'
#…等价于:
if len(unique_characters) > 10:
    raise AssertionError('Too many letters'

2.找出不同字母

>>> words = ['SEND', 'MORE', 'MONEY']
>>> ''.join(words)                   #join 字符串连接符
'SENDMOREMONEY'
>>> set(''.join(words))              #set 返回不重复字符
{'E', 'D', 'M', 'O', 'N', 'S', 'R', 'Y'}

3. List Set Tuple

>>> unique_characters = {'E', 'D', 'M', 'O', 'N', 'S', 'R', 'Y'}
>>> tuple(ord(c) for c in unique_characters)
(89, 83, 82, 77, 79, 78, 69, 68)
>>> set(ord(c) for c in unique_characters)
{68, 69, 77, 78, 79, 82, 83, 89}
>>> list(ord(c) for c in unique_characters)
[89, 83, 82, 77, 79, 78, 69, 68]

4. 排列

list(itertools.permutations('ABC', 3))    #取ABC 3个元素的排列组合
[('A', 'B', 'C'), ('A', 'C', 'B'),
 ('B', 'A', 'C'), ('B', 'C', 'A'),
 ('C', 'A', 'B'), ('C', 'B', 'A')]

5. 排列分组

>>> names = ['Alex', 'Anne', 'Chris', 'Dora', 'Ethan','John', 'Lizzie', 'Mike', 'Sarah', 'Wesley']
>>> import itertools
>>> groups = itertools.groupby(names,len)
>>> groups
<itertools.groupby object at 0x01433F30>
>>> list(groups)
[(4, <itertools._grouper object at 0x013DA830>),
 (5, <itertools._grouper object at 0x01426190>),
 (4, <itertools._grouper object at 0x01426C70>),
 (5, <itertools._grouper object at 0x01426130>),
 (4, <itertools._grouper object at 0x01426030>),
 (6, <itertools._grouper object at 0x014261D0>),
 (4, <itertools._grouper object at 0x014265D0>),
 (5, <itertools._grouper object at 0x01426110>),
 (6, <itertools._grouper object at 0x01426150>)]
>>> names = sorted(names,key=len) #列表需要排序才能group by
>>> names
['Alex',
 'Anne',
 'Dora',
 'John',
 'Mike',
 'Chris',
 'Ethan',
 'Sarah',
 'Lizzie',
 'Wesley']
>>> A = itertools.groupby(names,len)
>>> list(A)  #调用list() 函数会“耗尽”这个迭代器, 也就是说 你生成了迭代器中所有元素才创造了这个列表
[(4, <itertools._grouper object at 0x014261D0>),
 (5, <itertools._grouper object at 0x014268D0>),
 (6, <itertools._grouper object at 0x01426C90>)]
>>> for name_len, name_iter in A:  #List(A) 为空
...     print('%d' %(name_len))
...     for name in name_iter:
...         print(name)
...         
>>> 
>>> A = itertools.groupby(names,len) #重新产生
>>> for name_len, name_iter in A:
...     print('%d' %(name_len))
...     for name in name_iter:
...         print(name)
...         
4
Alex
Anne
Dora
John
Mike
5
Chris
Ethan
Sarah
6
Lizzie
Wesley

 6. List 的extend 和append的差别

>>> a_list = ['a','b','c']
>>> a_list.extend(['d','e']) #extend() 方法只接受一个参数，而该参数总是一个列表
>>> a_list
['a', 'b', 'c', 'd', 'e']
>>> len(a_list)
5
>>> a_list.append(['f','g']) #append() 方法只接受一个参数，但可以是任何数据类型
>>> a_list
['a', 'b', 'c', 'd', 'e', ['f', 'g']]
>>> len(a_list)
6

7. Set 的discard和remove的差别

>>> a_set = {1, 3, 6, 10, 15, 21, 28, 36, 45}
>>> a_set
{1, 3, 36, 6, 10, 45, 15, 21, 28}
>>> a_set.discard(10)                        ①
>>> a_set
{1, 3, 36, 6, 45, 15, 21, 28}
>>> a_set.discard(10)                        ②
>>> a_set
{1, 3, 36, 6, 45, 15, 21, 28}
>>> a_set.remove(21)                         ③
>>> a_set
{1, 3, 36, 6, 45, 15, 28}
>>> a_set.remove(21)                         ④
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 21
#区别在于：如果该值不在集合中，remove() 方法引发一个 KeyError 例外

8.混合字典 

SUFFIXES = {1000: ['KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'],
            1024: ['KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB']}
>>> SUFFIXES[1024]     ④
['KiB', 'MiB', 'GiB', 'TiB', 'PiB', 'EiB', 'ZiB', 'YiB']
>>> SUFFIXES[1000][3]  ⑤ #
'TB'

9.文件路径操作

>>> import os
>>> print(os.getcwd()) #当前工作路径
D:\study\workspace\pythonTest\src
>>> print(os.path.expanduser('~')) #$home路径
C:\Documents and Settings\yfzhou
>>> print(os.path.join(os.path.expanduser('~'),'hello','world','python.py')) #构建动态的文件夹和文件
... 
C:\Documents and Settings\yfzhou\hello\world\python.py

10. 罗列目录下的文件

>>> os.chdir('/Users/pilgrim/diveintopython3/') #改变当前路径
>>> import glob
>>> glob.glob('examples/*.xml')  #返回匹配的文件
['examples\\feed-broken.xml',
'examples\\feed-ns0.xml',
'examples\\feed.xml']
>>> os.chdir('examples/')  #改变路径
>>> glob.glob('*test*.py')  #返回匹配的通配符文件
['alphameticstest.py',
'pluraltest1.py'
]

11.文件元信息

>>> metadata = os.stat('roman.py') #用os.stat() 函数返回一个包含多种文件元信息的对象
>>> metadata.st_mtime
1373272096.4869184
>>> import time
>>> time.localtime(metadata.st_mtime)
time.struct_time(tm_year=2013, tm_mon=7, tm_mday=8, tm_hour=16, tm_min=28, tm_sec=16, tm_wday=0, tm_yday=189, tm_isdst=0) #可读性更强
>>> metadata.st_size #文件大小
2546
>>> print(os.path.realpath('roman.py')) #文件绝对路径
D:\study\workspace\pythonTest\src\roman.py

12.字典键值互换

>>> a_dict = {'a': 1, 'b': 2, 'c': 3}
>>> {value:key for key, value in a_dict.items()}
{1: 'a', 2: 'b', 3: 'c'}

 13. __str__ and __repr__

• __str__ is tried first for the print operation and the str built-in function (the internal

equivalent of which print runs). It generally should return a user-friendly
display.

• __repr__ is used in all other contexts: for interactive echoes, the repr function, and

nested appearances, as well as by print and str if no __str__ is present. It should
generally return an as-code string that could be used to re-create the object, or a
detailed display for developers

>>> class addboth(adder):
... def __str__(self):
... return '[Value: %s]' % self.data # User-friendly string
... def __repr__(self):
... return 'addboth(%s)' % self.data # As-code string
...
>>> x = addboth(4)
>>> x + 1
>>> x # Runs __repr__
addboth(5)
>>> print(x) # Runs __str__
[Value: 5]
>>> str(x), repr(x)
('[Value: 5]', 'addboth(5)')

Two usage notes:

1.keep in mind that __str__ and__repr__ must both return strings

2.depending on a container’s string-conversion logic, the user-friendly display of __str__ might only applywhen objects appear at the top level of a print operation; objects nested in larger objects might still print with their __repr__ or its default

>>> class Printer:
... def __init__(self, val):
... self.val = val
... def __str__(self): # Used for instance itself
... return str(self.val) # Convert to a string result
...
>>> objs = [Printer(2), Printer(3)]
>>> for x in objs: print(x) # __str__ run when instance printed
... # But not when instance in a list!
2
3
>>> print(objs)
[<__main__.Printer object at 0x025D06F0>, <__main__.Printer object at ...more...
>>> objs
[<__main__.Printer object at 0x025D06F0>, <__main__.Printer object at ...more...

----------------------------
>>> class Printer:
... def __init__(self, val):
... self.val = val
... def __repr__(self): # __repr__ used by print if no __str__
... return str(self.val) # __repr__ used if echoed or nested
...
>>> objs = [Printer(2), Printer(3)]
>>> for x in objs: print(x) # No __str__: runs __repr__
...
23
>>> print(objs) # Runs __repr__, not ___str__
[2, 3]
>>> objs
[2, 3]