Loop to remove characters from list not working

Question

I have a list of punctuation that I want the loop to remove from a user inputted sentence, and it seems to ignore multiple punctuation in a row?

punctuation = ['(', ')', '?', ':', ';', ',', '.', '!', '/', '"', "'"]
usr_str=input('Type in a line of text: ')

#Convert to list
usr_list= list(usr_str)

#Checks if each item in list is punctuation, and removes ones that are
for char in usr_list:
    if char in punctuation:
        usr_list.remove(char)

#Prints resulting string
print(''.join(usr_list))

Now this works for:

This is an example string, which works fine!

Which prints:

This is an example string which works fine

However, something like this:

Testing!!!!, )

Gives:

Testing!!

Thanks in advance for the help!

The problem is that the iterator isn't being moved back to check the character that takes the place of the one removed. However, you really should be using a cleaner approach such as a RegEx. — Spencer Ruport, Oct 18 '13 at 02:19
Ah okay Spencer that makes sense, and I am reasonably new at Python, so I will do some looking into Regular Expressions. Thanks! — Stensby, Oct 18 '13 at 02:22
If you don't want to get into a RegEx I'd also recommend @Soon's answer for `string.translate`. — Spencer Ruport, Oct 18 '13 at 20:36
See [Loop “Forgets” to Remove Some Items](http://stackoverflow.com/q/17299581/1730674) also [str.replace issue](http://stackoverflow.com/q/15456639/1730674) and [Remove specific characters from a string](http://stackoverflow.com/q/19458344/1730674) — askewchan, Oct 18 '13 at 20:41

Tim Peters · Accepted Answer · 2013-10-18T02:30:26.067

2

You're changing the list while you're iterating over it. Never a good idea.

One way to get it to work is to iterate over a copy of the list:

for char in usr_list[:]:  # this is the only part that changed; add [:] to make a copy
    if char in punctuation:
        usr_list.remove(char)

With more experience, you'll probably use a "list comprehension" instead:

usr_list = [char for char in usr_list if char not in punctuation]

You'll probably get other answers using filter() or regular expressions or ... too. But keep it simple at first :-)

Another way is to make two different lists. Say your input list is input_list. Then:

no_punc = []
for char in usr_list:
    if char not in punctuation:
        no_punc.append(char)

And note that there's really no need to use an input list in this way either. You could iterate directly over your usr_str instead.

edited Oct 18 '13 at 02:30

answered Oct 18 '13 at 02:24

Tim Peters

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Thanks so much Tim, this seems like a straightforward fix, and I understand both ways. My assignment specifies using a loop, so the first way will have to be what I use, but its nice to know there are simpler ways! – Stensby Oct 18 '13 at 02:26
Using `[:]` may be too obscure, though, so I just added another way - it will be even more obvious to you ;-) And added another way getting rid of one of the lists. – Tim Peters Oct 18 '13 at 02:30

awesoon · Answer 2 · 2013-10-19T03:36:08.523

2

This could be done using str.translate method:

In [10]: 'Testing!!!!, )'.translate(str.maketrans('', '', string.punctuation))
Out[10]: 'Testing '

edited Oct 19 '13 at 03:36

answered Oct 18 '13 at 02:26

awesoon

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Loop to remove characters from list not working

2 Answers2