How can I remove non-ASCII characters but leave periods and spaces?

Question

I'm working with a .txt file. I want a string of the text from the file with no non-ASCII characters. However, I want to leave spaces and periods. At present, I'm stripping those too. Here's the code:

def onlyascii(char):
    if ord(char) < 48 or ord(char) > 127: return ''
    else: return char

def get_my_string(file_path):
    f=open(file_path,'r')
    data=f.read()
    f.close()
    filtered_data=filter(onlyascii, data)
    filtered_data = filtered_data.lower()
    return filtered_data

How should I modify onlyascii() to leave spaces and periods? I imagine it's not too complicated but I can't figure it out.

Solution 1

You can filter all characters from the string that are not printable using string.printable, like this:

>>> s = "some\x00string. with\x15 funny characters"
>>> import string
>>> printable = set(string.printable)
>>> filter(lambda x: x in printable, s)
'somestring. with funny characters'

string.printable on my machine contains:

0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~ \t\n\r\x0b\x0c

EDIT: On Python 3, filter will return an iterable. The correct way to obtain a string back would be:

''.join(filter(lambda x: x in printable, s))

Solution 2

An easy way to change to a different codec, is by using encode() or decode(). In your case, you want to convert to ASCII and ignore all symbols that are not supported. For example, the Swedish letter å is not an ASCII character:

    >>>s = u'Good bye in Swedish is Hej d\xe5'
    >>>s = s.encode('ascii',errors='ignore')
    >>>print s
    Good bye in Swedish is Hej d

Edit:

Python3: str -> bytes -> str

>>>"Hej då".encode("ascii", errors="ignore").decode()
'hej d'

Python2: unicode -> str -> unicode

>>> u"hej då".encode("ascii", errors="ignore").decode()
u'hej d'

Python2: str -> unicode -> str (decode and encode in reverse order)

>>> "hej d\xe5".decode("ascii", errors="ignore").encode()
'hej d'

Solution 3

According to @artfulrobot, this should be faster than filter and lambda:

import re
re.sub(r'[^\x00-\x7f]',r'', your-non-ascii-string) 

See more examples here Replace non-ASCII characters with a single space

Solution 4

You may use the following code to remove non-English letters:

import re
str = "123456790 ABC#%? .(朱惠英)"
result = re.sub(r'[^\x00-\x7f]',r'', str)
print(result)

This will return

123456790 ABC#%? .()

Solution 5

Your question is ambiguous; the first two sentences taken together imply that you believe that space and "period" are non-ASCII characters. This is incorrect. All chars such that ord(char) <= 127 are ASCII characters. For example, your function excludes these characters !"#$%&\'()*+,-./ but includes several others e.g. []{}.

Please step back, think a bit, and edit your question to tell us what you are trying to do, without mentioning the word ASCII, and why you think that chars such that ord(char) >= 128 are ignorable. Also: which version of Python? What is the encoding of your input data?

Please note that your code reads the whole input file as a single string, and your comment ("great solution") to another answer implies that you don't care about newlines in your data. If your file contains two lines like this:

this is line 1
this is line 2

the result would be 'this is line 1this is line 2' ... is that what you really want?

A greater solution would include:

1. a better name for the filter function than onlyascii

2. recognition that a filter function merely needs to return a truthy value if the argument is to be retained:

   def filter_func(char):
       return char == '\n' or 32 <= ord(char) <= 126
   # and later:
   filtered_data = filter(filter_func, data).lower()
   

View more solutions

Comments

[joaquin]

chr(127) in string.printable ?

[joaquin]

what's up with those printable chars that are below ordinal 48 ?

[jterrace]

chr(127) in string.printable == False

[jterrace]

Do you mean 0b and 0c? They are part of string.whitespace.

[joaquin]

yes, and from the OP: if ord(char) < 48 or ord(char) > 127. About my second comment, I am refering to '*' ,'(', and other printable which are eliminated by the OP...

[jterrace]

Yeah, I was extrapolating that the OP probably meant all printable characters, rather than what was actually said, but might not be the case.

[jterrace]

Slightly simpler: lambda x: 32 <= ord(x) <= 126

[jterrace]

that's not the same as string.printable because it leaves out string.whitespace, although that might be what the OP wants, depends on things like \n and \t.

[joaquin]

@jterrace right, includes space (ord 32) but no returns and tabs

[jterrace]

yeah, just commenting on "this is equivalent to string.printable", but not true

[joaquin]

I edited the answer, thanks! the OP question is misleading if you do not read it carefully.

[Admin]

Thanks! I understand now. Sorry for the confusion - jterrace correctly interpreted my question.

[rickcnagy]

this is also great for just filtering to digits - filter(lambda x: x in string.digits, s)

[Xodarap777]

I get UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 27

[Xodarap777]

This is incredibly slow in a large file. Any suggestions?

[Xodarap777]

This answer is very helpful to those of us coming in to ask something similar to the OP, and your proposed answer is helpfully pythonic. I do, however, find it strange that there isn't a more efficient solution to the problem as you interpreted it (which I often run into) - character by character, this takes a very long time in a very large file.

[jterrace]

@Xodarap777 create a set(string.printable) and re-use it for the filtering. Also don't filter the whole file at once - do it in chunks of 8K-512K

[cjbarth]

The only problem with using filter is that it returns an iterable. If you need a string back (as I did because I needed this when doing list compression) then do this: ''.join(filter(lambda x: x in string.printable, s).

[undershock]

@cjbarth - comment is python 3 specific, but very useful. Thanks!

[Ben Liyanage]

I got that error when I put the actual unicode character in the string via copy paste. When you specify a string as u'thestring' encode works correctly.

[Noam Manos]

Why not use regular expression: re.sub(r'[^\x00-\x7f]',r'', your-non-ascii-string) . See this thread stackoverflow.com/a/20079244/658497

[gaborous]

This is the most compatible way of doing the OP's task, I tested in from Python 2.6 to Python 3.5.

[gaborous]

Works only on Py3, but it's elegant.

[artfulrobot]

@NoamManos this was 4-5 times faster for me thatn the join...filter...lambda solution, thanks.

[ShadowRanger]

I suspect changing lambda x: x in printable to printable.__contains__ would make it run faster; the lambda means more Python level code execution, while directly passing the built-in membership test method removes per character byte code execution.

[Jonny]

PyLint Complains on the use of filter when using the above code. Given that list comprehensions seem to be preferred would using ''.join(x for x in s if x in printable) be a) equivalent, and b) any better?

[Jonny]

Edit: I realise the above is a generator expression, but does the same apply?

[jterrace]

@Jonny - it's most likely equivalent, but I'd have to profile it to know for sure

[Spc_555]

For those who are getting the same error as @Xodarap777 : you should first .decode() the string, and only after that encode. For example s.decode('utf-8').encode('ascii', errors='ignore')

[Ctrl-C]

@Jonny, The result is the same, time differs (you need to compare if it happens to be a bottleneck). This is easier for an eye - the less the diversity of tools, the faster is reading comprehension. You may want to add an [Enter] before if and indent the second line so if starts just after ( from the first line.

[Danilo Souza Morães]

This solution answers OP's stated question, but beware that it won't remove non printable characters that are included in ASCII which I think is what OP intended to ask.

[Amon]

Am I the only one who this doesn't work for? Why wouldnt those characters be included in the printable list? like 0 or x for example?

[jterrace]

@CharlesSmith - those are escape sequences

[SherylHohman]

This would not allow for standard ASCII symbols, such as bullet points, degrees symbol, copyright symbol, Yen symbol, etc. Also, your first example includes non-printable symbols, such as BELL, which is undesirable.

[Brajesh]

when assigning value to a variable it works fine whereas reading from file has no effect on filtering.. Dont know why? any ideas?