Why Does My Python File Iteration Fail After the First Pass?

File Iteration Difficulties After Initial Pass

When programming, iterating through files is a common task. However, sometimes unexpected behavior can arise. In this case, iterating through a file in Python initially works but subsequently yields no output. This is exemplified by the following code:

import codecs
file = codecs.open('baby1990.html', 'r',encoding='utf-8', errors='ignore')
for line in file.readlines():
    print(line)

Upon running this code, the file's contents are correctly printed. However, a second attempt to iterate through the same file using for line in file.readlines(): produces no output.

This behavior stems from the nature of file iteration. When iterating through a file, the pointer responsible for reading the file advances until the end of the file is reached. In the initial iteration, the pointer progresses from the start of the file to the end. When attempting to iterate a second time, the pointer is still at the end of the file, and there is no more data to read.

To resolve this issue, the file pointer must be reset to the beginning. This can be achieved either by using the f.seek(0) method to explicitly reposition the pointer or by closing and reopening the file.

Alternatively, you can employ Python's with statement, which automatically closes a file after its execution, ensuring that the file pointer is reset. An example would be:

with codecs.open('baby1990.html', 'r',encoding='utf-8', errors='ignore') as file:  
    for line in file.readlines():  
        print(line)  

This code allows for multiple iterations without encountering the same issue.

The above is the detailed content of Why Does My Python File Iteration Fail After the First Pass?. For more information, please follow other related articles on the PHP Chinese website!