Python regular expression NoneType error

Question

I have a text file named "filename.txt"

content of file:

    This is just a text
    content to store
    in a file.

i have made two python scripts to extract "to" from the text file

my 1st script:

     #!/usr/bin/python
     import re
     f = open("filename.txt","r")
     for line in f:
              text = re.match(r"content (\S+) store",line)
              x = text.group(1)
              print x

my 2nd script:

    #!/usr/bin/python
    import re
    f = open("filename.txt","r")
    for line in f:
             text = re.match(r"content (\S+) store",line)
             if text:
                    x = text.group(1)
                    print x

2nd script gives the correct output

 bash-3.2$ ./script2.py
 to

but 1st script gives me an error

bash-3.2$ ./script1.py
Traceback (most recent call last):
File "./script1.py", line 6, in ?
x = text.group(1)
AttributeError: 'NoneType' object has no attribute 'group'

how is that adding an "if" condition gives me the correct output and when i remove it i get an error?

Robin · Accepted Answer · 2014-04-28 12:07:27Z

1

The error is pretty self-explanatory to me: re.match returns None if no match is found (see doc).

So when your regex doesn't match (eg first line), you're trying to access the group property of a NoneType object, it throws an error.

In the other case, you only access the property if text isn't None (since this is what the if text: checks, among other things).

edited Apr 28, 2014 at 12:07

answered Apr 28, 2014 at 12:00

Robin

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Comments

sshashank124 · Accepted Answer · 2014-04-28 11:59:14Z

1

This is because in your first code, your regex fails to match anything and therefore text is a NoneType. When you try to do group it throws the AttributeError: 'NoneType' object has no attribute 'group' error

However, for your regex, your code doesn't fail because you are careful to call group only if something was actually matched

Your second method is better since it's fail proof unlike the first one.

answered Apr 28, 2014 at 11:59

sshashank124

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