Python: How to ignore #comment lines when reading in a file

Question

In Python, I have just read a line form a text file and I'd like to know how to code to ignore comments with a hash # at the beginning of the line.

I think it should be something like this:

for     if line !contain #       then ...process line    else end for loop  

But I'm new to Python and I don't know the syntax

Answer 1

you can use startswith()

eg

for line in open("file"):     li=line.strip()     if not li.startswith("#"):         print line.rstrip() 

Answer 2

I recommend you don't ignore the whole line when you see a # character; just ignore the rest of the line. You can do that easily with a string method function called partition:

with open("filename") as f:     for line in f:         line = line.partition('#')[0]         line = line.rstrip()         # ... do something with line ... 

partition returns a tuple: everything before the partition string, the partition string, and everything after the partition string. So, by indexing with [0] we take just the part before the partition string.

EDIT: If you are using a version of Python that doesn't have partition(), here is code you could use:

with open("filename") as f:     for line in f:         line = line.split('#', 1)[0]         line = line.rstrip()         # ... do something with line ... 

This splits the string on a '#' character, then keeps everything before the split. The 1 argument makes the .split() method stop after a one split; since we are just grabbing the 0th substring (by indexing with [0]) you would get the same answer without the 1 argument, but this might be a little bit faster. (Simplified from my original code thanks to a comment from @gnr. My original code was messier for no good reason; thanks, @gnr.)

You could also just write your own version of partition(). Here is one called part():

def part(s, s_part):     i0 = s.find(s_part)     i1 = i0 + len(s_part)     return (s[:i0], s[i0:i1], s[i1:]) 

@dalle noted that '#' can appear inside a string. It's not that easy to handle this case correctly, so I just ignored it, but I should have said something.

If your input file has simple enough rules for quoted strings, this isn't hard. It would be hard if you accepted any legal Python quoted string, because there are single-quoted, double-quoted, multiline quotes with a backslash escaping the end-of-line, triple quoted strings (using either single or double quotes), and even raw strings! The only possible way to correctly handle all that would be a complicated state machine.

But if we limit ourselves to just a simple quoted string, we can handle it with a simple state machine. We can even allow a backslash-quoted double quote inside the string.

c_backslash = '\\' c_dquote = '"' c_comment = '#'   def chop_comment(line):     # a little state machine with two state varaibles:     in_quote = False  # whether we are in a quoted string right now     backslash_escape = False  # true if we just saw a backslash      for i, ch in enumerate(line):         if not in_quote and ch == c_comment:             # not in a quote, saw a '#', it's a comment.  Chop it and return!             return line[:i]         elif backslash_escape:             # we must have just seen a backslash; reset that flag and continue             backslash_escape = False         elif in_quote and ch == c_backslash:             # we are in a quote and we see a backslash; escape next char             backslash_escape = True         elif ch == c_dquote:             in_quote = not in_quote      return line 

I didn't really want to get this complicated in a question tagged "beginner" but this state machine is reasonably simple, and I hope it will be interesting.