How to remove a string from a list that startswith prefix in python

Question

I have this list of strings and some prefixes. I want to remove all the strings from the list that start with any of these prefixes. I tried:

prefixes = ('hello', 'bye')
list = ['hi', 'helloyou', 'holla', 'byeyou', 'hellooooo']
for word in list:
    list.remove(word.startswith(prexixes)

So I want my new list to be:

list = ['hi', 'holla']

but I get this error:

ValueError: list.remove(x): x not in list

What's going wrong?

Greg Hewgill · Answer 1 · 2014-04-30T22:42:17.547

35

You can create a new list that contains all the words that do not start with one of your prefixes:

newlist = [x for x in list if not x.startswith(prefixes)]

The reason your code does not work is that the startswith method returns a boolean, and you're asking to remove that boolean from your list (but your list contains strings, not booleans).

Note that it is usually not a good idea to name a variable list, since this is already the name of the predefined list type.

edited Apr 30 '14 at 22:42

answered Apr 30 '14 at 22:36

Greg Hewgill

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Hey, I learned that `startswith` can take a tuple as a parameter. Cool! – Greg Hewgill Apr 30 '14 at 22:42
I didn't actually use list as a variable name, it was just as an example here. This worked for me btw :) – EerlijkeDame Apr 30 '14 at 22:49
And yes startswith can take tuples as a parameter as of python 2.5 :) – EerlijkeDame Apr 30 '14 at 22:51
Thanks, I should use not more :) ...... NOT (haha, sorry for my poor sense of humor) – David Miedema Sep 14 '20 at 09:06

score 9 · Accepted Answer · answered Apr 30 '14 at 22:37

9

Greg's solution is definitely more Pythonic, but in your original code, you perhaps meant something like this. Observe that we make a copy (using list[:] syntax) and iterate over the copy, because you should not modify a list while iterating over it.

prefixes = ('hello', 'bye')
list = ['hi', 'helloyou', 'holla', 'byeyou', 'hellooooo']
for word in list[:]:
    if word.startswith(prefixes):
        list.remove(word)
print list

answered Apr 30 '14 at 22:37

merlin2011

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Even though Gregs answer may heve been more pythonic, I accapted this answer, because ths was precisely what I was looking for. Thanks! – EerlijkeDame Apr 30 '14 at 22:50
Hey, thanks for that answer. Sorry for digging out this old post, but could you explain why this produces the wrong result when using `for word in list:` instead of `for word in list[:]:` – nik.yan Aug 28 '21 at 10:30
@nik.yan I am very curious about this as well. I tested running `for word in list` and noticed that some of the strings that contained a prefix weren't being removed. However, when I tried `for word in list[:]`, all the correct string were being removed. – neuron Jun 01 '22 at 13:25
I think it's because Python internally tracks the index for the loop, and when you remove an element, the index isn't updated so it skips over items. – merlin2011 Jun 02 '22 at 05:48

score 0 · Answer 3 · answered Jun 01 '15 at 18:42

0

print len([i for i in os.listdir('/path/to/files') if not i.startswith(('.','~','#'))])

answered Jun 01 '15 at 18:42

David J Merritt

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How to remove a string from a list that startswith prefix in python

3 Answers3