Define custom String method

Question

The "String Method" as shown in, for example:

teststring = "[Syllabus]Sociology.131.AC.Race.and.Ethnicity.in.the.United.States (Spring 2012).docx"
print teststring.replace('.', ' ')

"[Syllabus]Sociology 131 AC Race and Ethnicity in the United States (Spring 2012) docx"

is fantastic and the script I'm writing involves A LOT of text manipulation, it's a lot of what I'm doing and as I add features it continues to be important.

One of the common manipulations I'm doing is:

teststring = "[Syllabus]Sociology.131.AC.Race.and.Ethnicity.in.the.United.States (Spring 2012).docx"
def f_type(f): return f.split('/')[-1] if os.path.isdir(f) else re.split(r'(\[Syllabus\]|\[Syllabus \d+\]|\[Video\]|)(.*?)( \(Fall \d+\)| \(Spring \d+\)| \(Summer \d+\)|)(\.part\d+\.rar$|\.\w+$)', f.split('/')[-1])[1]
def f_name(f): return f.split('/')[-1] if os.path.isdir(f) else re.split(r'(\[Syllabus\]|\[Syllabus \d+\]|\[Video\]|)(.*?)( \(Fall \d+\)| \(Spring \d+\)| \(Summer \d+\)|)(\.part\d+\.rar$|\.\w+$)', f.split('/')[-1])[2]
def f_term(f): return f.split('/')[-1] if os.path.isdir(f) else re.split(r'(\[Syllabus\]|\[Syllabus \d+\]|\[Video\]|)(.*?)( \(Fall \d+\)| \(Spring \d+\)| \(Summer \d+\)|)(\.part\d+\.rar$|\.\w+$)', f.split('/')[-1])[3]
def f_ext(f): return f.split('/')[-1] if os.path.isdir(f) else re.split(r'(\[Syllabus\]|\[Syllabus \d+\]|\[Video\]|)(.*?)( \(Fall \d+\)| \(Spring \d+\)| \(Summer \d+\)|)(\.part\d+\.rar$|\.\w+$)', f.split('/')[-1])[4]
print f_type(teststring)
print f_name(teststring)
print f_term(teststring)
print f_ext(teststring)

[Syllabus] Sociology.131.AC.Race.and.Ethnicity.in.the.United.States (Spring 2012) .docx

But I'd like to be able to add: ".ftype()", ".fname()", ".fterm()", and ".fext()" methods (corresponding to these functions I have). And I have no idea how to do that.

I'd be looking to use it in a bunch of different functions in the script (so it wouldn't be class-bound or anything).

I can't even figure out what I should be google-ing. But how can I add these methods?

P.S. The names of the methods aren't really important--so if I have to change these names to keep from conflicting with built-in methods or something that's okay.

EDIT: I'm looking to be able to use this method for things like:

def testfunction(f1, f2): print 'y' if f1.ftype()==f2.ftype() else 'n'

So I don't want it to be bound to one string or anything, I want to be able to use it for different strings.

kindall · Accepted Answer · 2012-05-17 16:36:57Z

4

You can't add methods to built-in types like str.

You can, however, create a subclass of str and add the methods you want. As a bonus, add @property so you don't need to call the method to get the value.

class MyString(str):
    @property
    def f_type(f):
        return f.split('/')[-1] if os.path.isdir(f) else re.split(r'(\[Syllabus\]|\[Syllabus \d+\]|\[Video\]|)(.*?)( \(Fall \d+\)| \(Spring \d+\)| \(Summer \d+\)|)(\.part\d+\.rar$|\.\w+$)', f.split('/')[-1])[1]

s = MyString(teststring)
s.f_type

Normally you'd use self as the name for the first parameter in a method's argument list (the one that receives the reference to the instance the method is attached to). In this case I just used f because your expression was already written to use it.

edited May 17, 2012 at 16:36

answered May 17, 2012 at 14:39

kindall

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2 Comments

jesse Over a year ago

But this isn't going to let me use if a_ftype()==b_ftype(): do_something

kindall Over a year ago

Sure it will, assuming you mean a.f_type == b.f_type. a and b just need to be your str subclass rather than regular strings, and this is easily accomplished.

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