如何将一行从.txt文件复制到经过修改的新.txt文件中

Question

I'm trying to copy lines in a .txt file and write them into a new .txt file with minor changes. For example, if the file reads 0 is less than 1 , I want to copy that into a new one but say ZERO is less than ONE . I've been able to create a new file but nothing gets written into it.

def numbers(fileName):
file = open(fileName, "r")
newFile = 'converted.txt'
converted = open(newFile, "w")
for line in file:
    if "0" in line:
        line.replace("0", "ZERO")
    elif "1" in line:
        line.replace("1", "ONE")
    else:
        return
return

Answer 1

There are two big problems with your code:

First, line.replace won't do anything to line itself. As the docs say, it will:

Return a copy of the string with all occurrences of substring old replaced by new…

But you're not storing that new string, or doing anything else with it.

Second, you never write anything to converted .

To fix both at once:

for line in file:
    if '0' in line:
        converted.write(line.replace('0', 'ZERO'))
    elif '1' in line:
        converted.write(line.replace('1', 'ZERO'))
    else:
        return

However, you also have a number of small problems. You return the first time you find a line with no 0 s or 1 s. If a line has both 0 s and 1 s, you will only replace the 0 s. You never close the file, which means the file may never get flushed to disk, and could end up empty or incomplete. So, let's fix all of those problems as well:

with open(fileName, "r") as file, open('converted.txt', 'w') as converted:
    for line in file:
        line = line.replace("0", "ZERO")
        line = line.replace("1", "ONE")
        converted.write(line)

It's perfectly safe to replace all the 0s even if there aren't any—it just won't do anything. (If you were trying to optimize things by skipping the expensive work if there was no work to do, the "0" in line takes just as long as the replace when there's nothing to do, so you've actually pessimized things… which is a good lesson to learn early in your programming career.) This means you don't need the if statements at all, you don't have to fix the way you've chained them up, and you don't have the problem with the return in the else .

And the with statement automatically calls close on both file and converted for you as soon as you leave it (even if you leave early because of, say, an unexpected exception).

Answer 2

1. Use with when opening files to automatically clean them up.
2. You never actually write anything to the file
3. Are you sure you want to return if the line doesn't have a 0 or 1 ?

-

def numbers(filename):
    with open(filename, 'rU') as file_in:
        with open('converted.txt', 'w') as file_out:
            for line in file_in:
                if '0' in line:
                    line = line.replace('0', 'ZERO')
                elif '1' in line:
                    line = line.replace('1', 'ONE')

                file_out.write(line)

Answer 3

Both TkTech and abarnet have some good advice in terms of how to open the file and save the return values. Automatic clean up is nice.

There is a logical issue afoot though. In the example sentence you gave, "0 is less than 1" that single line has both a "0" and a "1" in it. If you were to send that line through either function, it would modify the line to be "ZERO is less than 1" not your intended "ZERO is less than ONE." The reason for this is because the first if statement would catch the "0" but the use of elif or even else would indicate to the program "you already found the first condition, don't bother checking the rest." Since you want to catch any 1's regardless of whether or not any 0's exist, you shouldn't use any sort of else statement.

Here is how I would write it:

def convert(file):
    in_file = open(file, "r")
    out_file = open("/home/user/temp/converted.txt", "w+")
    # Read up on what r and w+ mean; docs.python.org

    for line in in_file:
        # Catch 0 in line
        if "0" in line:
            line = line.replace("0", "ZERO")

        # Catch 1 in line
        if "1" in line:
            line = line.replace("1", "ONE")

        # Write line to file
        out_file.write(line)
    #end for

    # Close open files; this is common practice in C
    # Not needed in Python when using "with open(file, 'r') as filename:"
    in_file.close()

    # Force it to write out anything left in buffer; not usually needed as Python usually does this when calling close()
    out_file.flush()
    out_file.close()

There are a number of improvements that could be made to my code, in addition to improvements that could be made depending on the format of the intended input file.

1. I could use TkTech's suggestion and then I wouldn't need to close anything at the end.
2. If I wanted to look for more than just a couple things I could use a dictionary or other data structure to store all the intended changes.

It does, however work in it's current state taking this:

0 is less than 1
1 is greater than 0
0 1
1     0

and giving me this:

ZERO is less than ONE
ONE is greater than ZERO
ZERO ONE
ONE     ZERO

A file more complex than this may give you unintended results, such as:

0 is less than 1 but larger than -1 ==> ZERO is less than ONE but larger than -ONE
0 is larger than -1 ==> ZERO is larger than -ONE
2 is larger than 0 ==> 2 is larger than ZERO
0 1 2 3 4 5 6 7 8 9 10 ==> ZERO ONE 2 3 4 5 6 7 8 9 ZEROONE
0000001 ==> ZEROZEROZEROZEROZEROZEROONE
1001101 ==> ONEZEROZEROONEONEZEROONE