如何查找文件名的特定部分/扩展名？

Question

I want to look for the extension of a filename, and the extension can only be 3 characters long.

it is something like,

filename = str(input("Please enter filename: "))

then I want to execute a task to look for the extension, and if the extension doesn't fulfill the requirement (ie 3 characters long), I'll add print "Invalid extension!" Else, print "Valid extension."

Answer 1

len(os.path.splitext('/foo/file.png')[1]) - 1 == 3

Answer 2

You can "cut" it like that if the extension is always 3 caracters :

$ python2
Python 2.7.2 (default, Jan 31 2012, 13:19:49) 
[GCC 4.6.2 20120120 (prerelease)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> x = "file.ext"
>>> print x[-3:]
ext

See Extracting extension from filename in Python too

Edit : You can rely on the mime type instead, depending of your needs, see How to find the mime type of a file in python?

Answer 3

This should do what you are describing:

filename = raw_input("Please enter filename: ") #This is already a string
if len(filename.split('.')[-1])==3:
   print "Valid extension"
else:
   print "Invalid extension!"

Hope this helps.