Python: How to ignore #comment lines in a File

# Python: How to ignore #comment lines in a File

To ignore #comment lines when reading a file in Python:

1. Use the with open() statement to open the file in reading mode.
2. Use a for loop to iterate over the lines in the file.
3. Check if each line doesn't start with a comment.

Suppose we have the following example.txt file.

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bobby
# hadz
#.com
one
# two
three

Here is the related Python script.

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with open('example.txt', 'r', encoding='utf-8') as file_obj:
    for line in file_obj:
        line = line.strip()
        if not line.startswith('#'):
            print(line)

Running the code sample produces the following output.

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bobby
one
three

The with open() statement takes care of closing the file after we're done reading from it.

We used a for loop to iterate over the lines in the file.

On each iteration, we used a negated call to the str.startswith() method to verify the string doesn't start with a hash #.

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if not line.startswith('#'):
    print(line)

Notice that we also used the str.strip() method.

The str.strip() method returns a copy of the string with the specified leading and trailing characters removed.

You can also use the continue statement to achieve the same result.

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with open('example.txt', 'r', encoding='utf-8') as file_obj:
    for line in file_obj:
        line = line.strip()
        if line.startswith('#'):
            continue
        print(line)

Running the code sample produces the following output.

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bobby
one
three

The continue statement continues with the next iteration of the loop.

If the line starts with a hash #, we simply skip to the next iteration.

If you also need to process the comment lines in a different way, use the else statement.

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with open('example.txt', 'r', encoding='utf-8') as file_obj:
    for line in file_obj:
        line = line.strip()
        if not line.startswith('#'):
            print(line)
        else:
            print(f'comment: {line}')

Running the code sample produces the following output.

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bobby
comment: # hadz
comment: #.com
one
comment: # two
three

# Only ignoring the text after the hash #

If you only want to ignore the text after the hash # and not necessarily the newline character, use the str.partition() method.

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with open('example.txt', 'r', encoding='utf-8') as file_obj:
    for line in file_obj:
        line = line.partition('#')[0]
        line = line.rstrip()

        print(line)

Running the code sample produces the following output.

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bobby


one

three

The str.partition method splits the string at the first occurrence of the supplied separator.

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# ('', '#', '.com')
print('#.com'.partition('#'))

The method returns a tuple containing 3 elements - the part before the separator, the separator, and the part after the separator.

# Python: How to ignore #comment lines in a File using shlex

You can also use the shlex class from the shlex module to ignore #comment lines in a file.

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from shlex import shlex

with open('example.txt', 'r', encoding='utf-8') as file_obj:
    for line in file_obj:
        lex_analyzer = shlex(line)
        lex_analyzer.whitespace = '\n'

        line = ''.join(list(lex_analyzer))

        if not line:
            continue

        print(line)

Running the code sample produces the following output.

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bobby
one
three

The shlex.shlex class creates a lexical analyzer object that is used for shell-like syntaxes.

I set the whitespace attribute to \n to remove the newlines.

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lex_analyzer.whitespace = '\n'

If you don't want to strip the newlines, set the attribute to an empty string.

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lex.whitespace = ''

If you need to get each line that doesn't contain a comment as a list of words, use the shlex.split() method.

Suppose we have the following example.txt file.

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bobby one two
# hadz abc
#.com def
three
# four
five six

Here is the related Python script.

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from shlex import split

with open('example.txt', 'r', encoding='utf-8') as file_obj:
    for line in file_obj:
        line = split(line, comments=True)
        print(line)

        if not line:
            continue

        print(line)

Running the code sample produces the following output.

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['bobby', 'one', 'two']
['bobby', 'one', 'two']
[]
[]
['three']
['three']
[]
['five', 'six']
['five', 'six']

The shlex.split() method splits the supplied string using a shell-like syntax.

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line = split(line, comments=True)

When the comments argument is set to True, the parsing of comments in the given string is enabled.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Remove the Extension from a Filename in Python
• How to save user input to a File in Python
• Write a List of Tuples to a File in Python
• Write a String to a File on a New Line every time in Python
• How to check if a File is Empty in Python
• Count number of unique Words in a String or File in Python