我如何使该程序将具有给定单词的每一行写入文件？

Question

i have everything working but the final step of having it write the lines to a file. any help would be awesome. i assume its either an issue with a function or a control structure thing.

def process(word, file_name, new_file_name):
    '''Check the file for the word and returns lines with the word in it
    '''

    file=open(file_name, 'r')
    file2=open(new_file_name, 'w')

    for line in file:
        if word in line:
               file2.write(line)    
    else:
        print("That word is not in this file.")

    file.close()
    print('File written')


def main(): 
    global line 

    word=input('Enter a word: ').strip().lower()
    file_name=input('Enter a file name: ')
    new_file_name=input('Enter a new file name: ')
    process(word, file_name, new_file_name)



main()

Answer 1

You forgot to close file2 . Close that and it should be written out.

Also, a better way to open and close files is usually to use the with context manager:

with open(file_path, 'w') as my_f:
    my_f.write('foo')

That will open and close on its own, closing outside of the with scope.

Also, it's more standard to write a main this way:

if __name__ == '__main__':
    main()
    # or just the main body here if it's simple enough

The reason for that is that you can import the file in other source code, and it won't execute main after it imports. You can write a module, import from it, or have test code in your main and then execute that directly to test it.

Also, what qwrrty said about your for else statement is correct. It will always print.

An else in a for or while loop is an interesting feature of Python that you won't see in many programming languages, for better or worse. It executes if the loop does not exit from a break statement. There is also a try except else that executes the else if the try block completes without any exception.

You should use an else for a situation like when you are iterating through a container, searching for an element, and want logic if you do not find it. If you find it, you are done, and you break . I don't see a good reason for an else in your example.

Answer 2

From briefly testing this code, it appears to work as intended, and writes to file2 all the lines from file that match word .

This code is printing a misleading message:

for line in file:
    if word in line:
           file2.write(line)    
else:
    print("That word is not in this file.")

The else clause to for is run when the iterator is exhausted, so this will always print "That word is not in this file." You want something more like this:

found = False
for line in file:
    if word in line:
        file2.write(line)
        found = True

if not found:
    print("That word is not in this file.")

As noted, it is also a good idea to make sure that you close file2 when you are done with it, and the with open(...) as file2 context manager is useful for that. But that should be causing the program to misbehave in this case.