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: string16  #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 filegoodbye 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 displaySpam43,44,45[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 int44>>> 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==s2True>>> s1 is s2True>>> s1='hello world'>>> s2='hello world'>>> s1 is s2False

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