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This article mainly introduces the python file read and write how to achieve the relevant knowledge, the content is detailed and easy to understand, the operation is simple and fast, has a certain reference value, I believe that everyone after reading this python document read and write how to achieve the article will have a harvest, let's take a look.

Read the file

Open a file with the open () method (open () returns a file object that can be iterated):

> f = open ('test.txt', 'r')

R indicates a text file and rb is a binary file. (the default value of this mode parameter is r)

If the file does not exist, the open () function throws an IOError error and gives you the error code and details to tell you that the file does not exist:

> f=open ('test.txt', 'r') Traceback (most recent call last): File "", line 1, in FileNotFoundError: [Errno 2] No such file or directory:' test.txt'

The file must be closed after use, because the file object will occupy the resources of the operating system, and the number of files that the operating system can open at the same time is limited.

> > f.close ()

Because IOError can be generated when a file is read or written, once an error occurs, the subsequent f.close () will not be called. So, to ensure that the file is closed correctly regardless of whether there is an error or not, we can use try. Finally to achieve:

Try: F = open ('/ path/to/file', 'r') print (f.read ()) finally: if f: f.close ()

But it's too tedious to write this every time, so Python introduces a with statement to automatically help us call the close () method:

With open ('/ path/to/file', 'r') as f: print (f.read ())

The python file object provides three read methods: read (), readline (), and readlines (). Each method can accept a variable to limit the amount of data read at a time.

Read () reads the entire file at a time, which is usually used to put the contents of the file in a string variable. If the file is larger than the available memory, you can repeatedly call the read (size) method to read up to size bytes at a time to be on the safe side.

The difference between readlines () is that the latter reads the entire file at once, like .read (). .readlines () automatically parses the contents of the file into a list of lines that can be created by Python's for... In... Structure for processing.

Readline () reads only one line at a time, which is usually much slower than readlines (). Readline () should be used only if there is not enough memory to read the entire file at once.

Note: these three methods also read the'\ n' at the end of each line. It does not remove the'\ n' by default, but requires us to remove it manually.

In [2]: with open ('test1.txt', 'r') as F1: list1 = f1.readlines () In [3]: list1Out [3]: [' 111\ n', '222\ n', '333\ n', '444\ n', '555\ n', '666\ n']

Remove'\ n'

In [4]: with open ('test1.txt', 'r') as F1: list1 = f1.readlines () for i in range (0, len (list1)): list1 [I] = List1 [I] .rstrip ('\ n') In [5]: list1Out [5]: ['111,' 222, '333,' 444, '555,' 666]

The'\ n'is also read in for read () and readline (), but it can be displayed normally when print (because the'\ n'in print is considered to mean line wrapping)

In [7]: with open ('test1.txt', 'r') as F1: list1 = f1.read () In [8]: list1Out [8]:' 111\ n222\ n333\ n444\ n666\ n'In [9]: print (list1) 111222333444555666In [10]: with open ('test1.txt', 'r') as F1: list1 = f1.readline () In [11]: list1Out [11]:' 111\ n'In [12]: print (list1) 111

An example of a python interview question:

There are two files, each with many lines of ip addresses, to find the same ip address in both files:

# coding:utf-8import bisectwith open ('test1.txt', 'r') as F1: list1 = f1.readlines () for i in range (0, len (list1)): list1 [I] = List1 [I] .strip ('\ n') with open ('test2.txt', 'r') as f2: list2 = f2.readlines () for i in range (0 Len (list2): list2 [I] = List2 [I] .strip ('\ n') list2.sort () length_2 = len (list2) same_data = [] for i in list1: pos = bisect.bisect_left (list2, I) if pos

< len(list2) and list2[pos] == i: same_data.append(i)same_data = list(set(same_data))print(same_data) 要点就是：（1）用with （2）处理行末的'\n' （3）使用二分查找提高算法效率。（4）使用set快速去重。 写文件 写文件和读文件是一样的，唯一区别是调用open()函数时，传入标识符'w'或者'wb'表示写文本文件或写二进制文件： >

> > f = open ('test.txt', 'w') # if' wb' means writing binaries > f.write ('Hello, worldview') > f.close ()

Note: the'w 'mode is obscure: if you don't have this file, create one; if so, empty the contents of the original file before writing something new. So if you don't want to empty the original content and just add new content to it, use the'a 'mode.

We can repeatedly call write () to write to the file, but be sure to call f.close () to close the file. When we write a file, the operating system often does not write the data to disk immediately, but caches it in memory and writes it slowly when we are free. Only when the close () method is called does the operating system guarantee that all unwritten data is written to disk. The consequence of forgetting to call close () is that only part of the data may be written to disk and the rest is lost. So, use the with statement to get insurance:

With open ('test.txt',') as f: f.write ('Hello, Worldwide')

The python file object provides two "write" methods: write () and writelines ().

The write () method, corresponding to the read () and readline () methods, writes the string to the file.

The writelines () method corresponds to the readlines () method and is also an operation on the list. It takes a list of strings as parameters, writes them to the file, and newline characters are not automatically added, so you need to explicitly add newline characters.

F1 = open ('test1.txt', 'w') f1.writelines (["1", "2", "3"]) # at this time the content of test1.txt is: 123f1 = open (' test1.txt', 'w') f1.writelines (["1\ n", "2\ n", "3\ n"]) # the content of test1.txt is: # # 2 # 3 about the mode parameter of open ():

'ringing: read

'wicked: write

'asides: append

'rust'= = rroomw (readable and writable, IOError if the file does not exist)

'Wendy'= = Whiter (readable and writable, create if the file does not exist)

'await'= awrr (appendable and writable, create a file if it doesn't exist)

Correspondingly, if it is a binary file, just add a b:

'rb' 'wb'' ab' 'rb+'' wb+' 'ab+'

File_obj.seek (offset,whence=0)

The file_obj.seek (offset,whence=0) method is used to move the file pointer in the file. Offset indicates how much the offset is. The optional parameter whence indicates where to start the offset. The default is 0 for the beginning of the file, 1 for the current location, and 2 for the end of the file. For example:

F = open ("test1.txt", "a +") print (f.read ()) f.write ('1') f.seek (0,0) # move the file pointer from the end to the beginning, without this sentence read () cannot read the correct thing print (f.read ()) f.close ()

Note: the change in the pointer of this file only works on'r', but not on'w 'and' a'. If it is'w', then write () will always be written from the beginning (it will overwrite the content in the following corresponding position), and if it is'a', write () will always be appended from the end.

Character coding

To read a non-UTF-8-encoded text file, you need to pass the encoding parameter to the open () function, for example, to read a GBK-encoded file:

> f = open ('test.txt', 'ritual, encoding='gbk') > f.read ()' Test'

When you encounter some files with irregular encoding, you may encounter UnicodeDecodeError because there may be some illegally encoded characters in the text file. In this case, the open () function also takes an errors parameter indicating what to do if a coding error is encountered. The easiest way is to simply ignore:

> I believe that everyone has a certain understanding of the knowledge of "how to read and write python documents". If you want to learn more knowledge, you are welcome to follow the industry information channel.

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