Write a program to delete comment lines from a file in Python

In this tutorial we will Write a program to delete comment lines from a file in python . We know that any text after “#” in Python is treated as comment . Now we will write a Python program to remove comments from a Python file .

To learn more about Python comments click here

Steps to remove comments from a Python file

• Open the required file in “r” mode using open() function
• Read all the lines from the file using readlines() method and store the list in a variable
• Iterate all lines of the list and check if the line contains “#” or not
• If the line starts with “#” then simply remove the line
• But if the line does not start with “#” then check when “#” is found is there any mismatch between “(” and “)” . If no mismatch found then delete the all character after “#” with “#”
• Now open a new file in “W” mode and using writelines() method write the changed list to the file .

We have saved the file as “oldfile.py” from which we want to remove comments in the same folder we are working now . Here is the contents of the file –

Python program to remove comments from a Python file

# reading the file
with open("oldfile.py") as fp:
    contents=fp.readlines()

# initialize two counter to check mismatch between "(" and ")"
open_bracket_counter=0
close_bracket_counter=0 

# whenever an element deleted from the list length of the list will be decreased
decreasing_counter=0   

for number in range(len(contents)):

    # checking if the line contains "#" or not
    if "#" in contents[number-decreasing_counter]:

        # delete the line if startswith "#"
        if contents[number-decreasing_counter].startswith("#"):
            contents.remove(contents[number-decreasing_counter])
            decreasing_counter+=1

        # delete the character after the "#"    
        else:  
            newline=""  
            for character in contents[number-decreasing_counter]:
                if character=="(":
                    open_bracket_counter+=1
                    newline+=character
                elif character==")":
                    close_bracket_counter+=1
                    newline+=character
                elif character=="#" and open_bracket_counter==close_bracket_counter:
                    break
                else:
                    newline+=character
            contents.remove(contents[number-decreasing_counter])     
            contents.insert(number-decreasing_counter,newline)   


# writing into a new file
with open("newfile.py","w") as fp:
    fp.writelines(contents)

Let’s have a look how the above code will delete comments from a Python file –

• First it will open the “oldfile.py” in “r” mode . After calling readlines() method it will return a list ['# This is comment1\n', 'print("Hello, #World!") # this is comment2\n', '\n', '# this is comment3'] which will be stored in contents variable .
• Then using for-loop we will check all the line(element) of the list if it contains “#” or not . We can see that 1st , 2nd and 4th line contains “#” .
• As 1st and 4th line starts with “#” so it is deleted by remove() method .
• But second line contains “#” in the middle . So we need to create a new variable (here it is newline) , iterate every character of the second line and add each character to the newline . When program finds first “#” then “(” starts but “)” does not end so loop will not stop . But when program finds second “#” then “(” starts and “)” ends so loop will break . Now we will remove the second line and add the newline in that position . So now contents will look like ['print("Hello, #World!") ', '\n']
• After that we will open “newfile.py” in “w” mode and write the contents using writelines() method .

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