谢谢大家，我找到了适合我问题的解决方案。我在http://code.activestate.com/recipes/173220/上找到了此代码，并做了一些修改以适合我。

它工作正常。

from __future__ import division
import string

def istext(filename):
    s=open(filename).read(512)
    text_characters = "".join(map(chr, range(32, 127)) + list("\n\r\t\b"))
    _null_trans = string.maketrans("", "")
    if not s:
        # Empty files are considered text
        return True
    if "\0" in s:
        # Files with null bytes are likely binary
        return False
    # Get the non-text characters (maps a character to itself then
    # use the 'remove' option to get rid of the text characters.)
    t = s.translate(_null_trans, text_characters)
    # If more than 30% non-text characters, then
    # this is considered a binary file
    if float(len(t))/float(len(s)) > 0.30:
        return False
    return True