Stripping non printable characters from a string in python

I use to run

$s =~ s/[^[:print:]]//g;

on Perl to get rid of non printable characters.

In Python there's no POSIX regex classes, and I can't write [:print:] having it mean what I want. I know of no way in Python to detect if a character is printable or not.

What would you do?

EDIT: It has to support Unicode characters as well. The string.printable way will happily strip them out of the output. curses.ascii.isprint will return false for any unicode character.

Solution 1:

Iterating over strings is unfortunately rather slow in Python. Regular expressions are over an order of magnitude faster for this kind of thing. You just have to build the character class yourself. The unicodedata module is quite helpful for this, especially the unicodedata.category() function. See Unicode Character Database for descriptions of the categories.

import unicodedata, re, itertools, sys

all_chars = (chr(i) for i in range(sys.maxunicode))
categories = {'Cc'}
control_chars = ''.join(c for c in all_chars if unicodedata.category(c) in categories)
# or equivalently and much more efficiently
control_chars = ''.join(map(chr, itertools.chain(range(0x00,0x20), range(0x7f,0xa0))))

control_char_re = re.compile('[%s]' % re.escape(control_chars))

def remove_control_chars(s):
    return control_char_re.sub('', s)

For Python2

import unicodedata, re, sys

all_chars = (unichr(i) for i in xrange(sys.maxunicode))
categories = {'Cc'}
control_chars = ''.join(c for c in all_chars if unicodedata.category(c) in categories)
# or equivalently and much more efficiently
control_chars = ''.join(map(unichr, range(0x00,0x20) + range(0x7f,0xa0)))

control_char_re = re.compile('[%s]' % re.escape(control_chars))

def remove_control_chars(s):
    return control_char_re.sub('', s)

For some use-cases, additional categories (e.g. all from the control group might be preferable, although this might slow down the processing time and increase memory usage significantly. Number of characters per category:

• Cc (control): 65
• Cf (format): 161
• Cs (surrogate): 2048
• Co (private-use): 137468
• Cn (unassigned): 836601

Edit Adding suggestions from the comments.

Solution 2:

As far as I know, the most pythonic/efficient method would be:

import string

filtered_string = filter(lambda x: x in string.printable, myStr)

Solution 3:

You could try setting up a filter using the unicodedata.category() function:

import unicodedata
printable = {'Lu', 'Ll'}
def filter_non_printable(str):
  return ''.join(c for c in str if unicodedata.category(c) in printable)

See Table 4-9 on page 175 in the Unicode database character properties for the available categories

Solution 4:

The following will work with Unicode input and is rather fast...

import sys

# build a table mapping all non-printable characters to None
NOPRINT_TRANS_TABLE = {
    i: None for i in range(0, sys.maxunicode + 1) if not chr(i).isprintable()
}

def make_printable(s):
    """Replace non-printable characters in a string."""

    # the translate method on str removes characters
    # that map to None from the string
    return s.translate(NOPRINT_TRANS_TABLE)


assert make_printable('Café') == 'Café'
assert make_printable('\x00\x11Hello') == 'Hello'
assert make_printable('') == ''

My own testing suggests this approach is faster than functions that iterate over the string and return a result using str.join.