How to delete an item in a list if it exists?


Question

I am getting new_tag from a form text field with self.response.get("new_tag") and selected_tags from checkbox fields with

self.response.get_all("selected_tags")

I combine them like this:

tag_string = new_tag
new_tag_list = f1.striplist(tag_string.split(",") + selected_tags)

(f1.striplist is a function that strips white spaces inside the strings in the list.)

But in the case that tag_list is empty (no new tags are entered) but there are some selected_tags, new_tag_list contains an empty string " ".

For example, from logging.info:

new_tag
selected_tags[u'Hello', u'Cool', u'Glam']
new_tag_list[u'', u'Hello', u'Cool', u'Glam']

How do I get rid of the empty string?

If there is an empty string in the list:

>>> s = [u'', u'Hello', u'Cool', u'Glam']
>>> i = s.index("")
>>> del s[i]
>>> s
[u'Hello', u'Cool', u'Glam']

But if there is no empty string:

>>> s = [u'Hello', u'Cool', u'Glam']
>>> if s.index(""):
        i = s.index("")
        del s[i]
    else:
        print "new_tag_list has no empty string"

But this gives:

Traceback (most recent call last):
  File "<pyshell#30>", line 1, in <module>
    if new_tag_list.index(""):
        ValueError: list.index(x): x not in list

Why does this happen, and how do I work around it?

1
211
4/5/2016 4:09:32 AM

Accepted Answer

1) Almost-English style:

Test for presence using the in operator, then apply the remove method.

if thing in some_list: some_list.remove(thing)

The removemethod will remove only the first occurrence of thing, in order to remove all occurrences you can use while instead of if.

while thing in some_list: some_list.remove(thing)    
  • Simple enough, probably my choice.for small lists (can't resist one-liners)

2) Duck-typed, EAFP style:

This shoot-first-ask-questions-last attitude is common in Python. Instead of testing in advance if the object is suitable, just carry out the operation and catch relevant Exceptions:

try:
    some_list.remove(thing)
except ValueError:
    pass # or scream: thing not in some_list!
except AttributeError:
    call_security("some_list not quacking like a list!")

Off course the second except clause in the example above is not only of questionable humor but totally unnecessary (the point was to illustrate duck-typing for people not familiar with the concept).

If you expect multiple occurrences of thing:

while True:
    try:
        some_list.remove(thing)
    except ValueError:
        break
  • a little verbose for this specific use case, but very idiomatic in Python.
  • this performs better than #1
  • PEP 463 proposed a shorter syntax for try/except simple usage that would be handy here, but it was not approved.

However, with contextlib's suppress() contextmanager (introduced in python 3.4) the above code can be simplified to this:

with suppress(ValueError, AttributeError):
    some_list.remove(thing)

Again, if you expect multiple occurrences of thing:

with suppress(ValueError):
    while True:
        some_list.remove(thing)

3) Functional style:

Around 1993, Python got lambda, reduce(), filter() and map(), courtesy of a Lisp hacker who missed them and submitted working patches*. You can use filter to remove elements from the list:

is_not_thing = lambda x: x is not thing
cleaned_list = filter(is_not_thing, some_list)

There is a shortcut that may be useful for your case: if you want to filter out empty items (in fact items where bool(item) == False, like None, zero, empty strings or other empty collections), you can pass None as the first argument:

cleaned_list = filter(None, some_list)
  • [update]: in Python 2.x, filter(function, iterable) used to be equivalent to [item for item in iterable if function(item)] (or [item for item in iterable if item] if the first argument is None); in Python 3.x, it is now equivalent to (item for item in iterable if function(item)). The subtle difference is that filter used to return a list, now it works like a generator expression - this is OK if you are only iterating over the cleaned list and discarding it, but if you really need a list, you have to enclose the filter() call with the list() constructor.
  • *These Lispy flavored constructs are considered a little alien in Python. Around 2005, Guido was even talking about dropping filter - along with companions map and reduce (they are not gone yet but reduce was moved into the functools module, which is worth a look if you like high order functions).

4) Mathematical style:

List comprehensions became the preferred style for list manipulation in Python since introduced in version 2.0 by PEP 202. The rationale behind it is that List comprehensions provide a more concise way to create lists in situations where map() and filter() and/or nested loops would currently be used.

cleaned_list = [ x for x in some_list if x is not thing ]

Generator expressions were introduced in version 2.4 by PEP 289. A generator expression is better for situations where you don't really need (or want) to have a full list created in memory - like when you just want to iterate over the elements one at a time. If you are only iterating over the list, you can think of a generator expression as a lazy evaluated list comprehension:

for item in (x for x in some_list if x is not thing):
    do_your_thing_with(item)

Notes

  1. you may want to use the inequality operator != instead of is not (the difference is important)
  2. for critics of methods implying a list copy: contrary to popular belief, generator expressions are not always more efficient than list comprehensions - please profile before complaining
619
8/16/2017 6:38:26 PM

try:
    s.remove("")
except ValueError:
    print "new_tag_list has no empty string"

Note that this will only remove one instance of the empty string from your list (as your code would have, too). Can your list contain more than one?


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