Removing all non-numeric characters from string in Python

Question

How do we remove all non-numeric characters from a string in Python?

Answer 1

Fastest approach, if you need to perform more than just one or two such removal operations (or even just one, but on a very long string!-), is to rely on the translate method of strings, even though it does need some prep:

>>> import string
>>> allchars = ''.join(chr(i) for i in xrange(256))
>>> identity = string.maketrans('', '')
>>> nondigits = allchars.translate(identity, string.digits)
>>> s = 'abc123def456'
>>> s.translate(identity, nondigits)
'123456'

The translate method is different, and maybe a tad simpler simpler to use, on Unicode strings than it is on byte strings, btw:

>>> unondig = dict.fromkeys(xrange(65536))
>>> for x in string.digits: del unondig[ord(x)]
... 
>>> s = u'abc123def456'
>>> s.translate(unondig)
u'123456'

You might want to use a mapping class rather than an actual dict, especially if your Unicode string may potentially contain characters with very high ord values (that would make the dict excessively large;-). For example:

>>> class keeponly(object):
...   def __init__(self, keep): 
...     self.keep = set(ord(c) for c in keep)
...   def __getitem__(self, key):
...     if key in self.keep:
...       return key
...     return None
... 
>>> s.translate(keeponly(string.digits))
u'123456'
>>>