Python 学习笔记（二）文件操作

Files

• File iterators are best for reading lines
• Content is strings, not objects
• close is usually optional
• Files are buffered and seekable.

>>> myfile = open('myfile.txt', 'w') # Open for text output: create/empty
>>> myfile.write('hello text file\n') # Write a line of text: string
16  #length
>>> myfile.write('goodbye text file\n')
18
>>> myfile.close() # Flush output buffers to disk
>>> myfile = open('myfile.txt') # Open for text input: 'r' is default
>>> myfile.readline() # Read the lines back
'hello text file\n'

>>> for line in open('myfile.txt'):
    print(line)


hello text file

goodbye text file

File写入任意类型

>>> X, Y, Z = 43, 44, 45 # Native Python objects
>>> S = 'Spam' # Must be strings to store in file
>>> D = {'a': 1, 'b': 2}
>>> L = [1, 2, 3]
>>>
>>> F = open('datafile.txt', 'w') # Create output file
>>> F.write(S + '\n') # Terminate lines with \n
>>> F.write('%s,%s,%s\n' % (X, Y, Z)) # Convert numbers to strings
>>> F.write(str(L) + '$' + str(D) + '\n') # Convert and separate with $
>>> F.close()

>>> chars = open('datafile.txt').read() # Raw string display
>>> chars
"Spam\n43,44,45\n[1, 2, 3]${'a': 1, 'b': 2}\n"
>>> print(chars) # User-friendly display
Spam
43,44,45
[1, 2, 3]${'a': 1, 'b': 2}

>>> F = open('datafile.txt') # Open again
>>> line = F.readline() # Read one line
>>> line
'Spam\n'
>>> line.rstrip() # Remove end-of-line
'Spam'

>>> int(parts[1]) # Convert from string to int
44
>>> numbers = [int(P) for P in parts] # Convert all in list at once
>>> numbers
[43, 44, 45]

>>> line = F.readline()
>>> parts = line.split('$')
>>> objects = [eval(P) for P in parts] # eval Convert to any object type
>>> objects
[[1, 2, 3], {'a': 1, 'b': 2}]

按行遍历文件：

with open('myfile.txt') as myfile:
    for line in myfile:
            print(line)

 Copy

>>> L = [1,2,3]
>>> D = {'a':1, 'b':2}

>>> A = L[:] # Instead of A = L (or list(L))
>>> B = D.copy() # Instead of B = D (ditto for sets)

Two examples:

- Change L will change M too

>>> L = [1, 2, 3]
>>> M = ['X', L, 'Y'] # Embed a reference to L
>>> M
['X', [1, 2, 3], 'Y']
>>> L[1] = 0 # Changes M too
>>> M
['X', [1, 0, 3], 'Y']

- Only change L, not M

>>> L = [1, 2, 3]
>>> M = ['X', L[:], 'Y'] # Embed a copy of L
>>> L[1] = 0 # Changes only L, not M
>>> L
[1, 0, 3]
>>> M
['X', [1, 2, 3], 'Y']

比较相等

• The == operator tests value equivalence.
• The is operator tests object identity.

•  Numbers are true if nonzero.
• Other objects are true if nonempty.

Python internally caches and reuses some strings as an optimization, there really is just a single string 'hello' in
memory：

>>> s1='hello'
>>> s2='hello'
>>> s1==s2
True
>>> s1 is s2
True
>>> s1='hello world'
>>> s2='hello world'
>>> s1 is s2
False

有意思的例子，注意嵌套的影响，tuple 与list 不同，tuple的Y与X 一样

>>> L = [4, 5, 6]
>>> X = L * 4 # Like [4, 5, 6] + [4, 5, 6] + ...
>>> Y = [L] * 4 # [L] + [L] + ... = [L, L,...]
>>> X
[4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6]
>>> Y
[[4, 5, 6], [4, 5, 6], [4, 5, 6], [4, 5, 6]]


>>> L[1] = 0 # Impacts Y but not X
>>> X
[4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6]
>>> Y
[[4, 0, 6], [4, 0, 6], [4, 0, 6], [4, 0, 6]]

How to change tuple (4,5,6) to (1,5,6)

>>> T = (4,5,6)
>>> T = (1,) + T[1:]
>>> T
(1, 5, 6)

换值

>>> X='a'
>>> Y='B'
>>> X,Y = Y,X
>>> X
'B'

赋值

>>> a, *b = 'spam' #New method in 3.0
>>> b
['p', 'a', 'm']

>>> L = [1,2,3,4]
>>> while L:
   　　 f,*L=L  #f = L[0]
  　　  print(f,L)

    
1 [2, 3, 4]
2 [3, 4]
3 [4]
4 []

赋空值，需单独

>>> a = b = []
>>> b.append(42)
>>> a, b
([42], [42])

>>> a = []
>>> b = []
>>> b.append(42)
>>> a, b
([], [42])

>>> L = [1, 2]
>>> L.append(3) # Append is an in-place change
>>> L
[1, 2, 3]

>>> L = L.append(4) # But append returns None, not L
>>> print(L) # So we lose our list!
None