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?
All you have to do is this
list = ["a", "b", "c"]
try:
list.remove("a")
except:
print("meow")
but that method has an issue. You have to put something in the except place so i found this:
list = ["a", "b", "c"]
if "a" in str(list):
list.remove("a")
Here's another one-liner approach to throw out there:
next((some_list.pop(i) for i, l in enumerate(some_list) if l == thing), None)
It doesn't create a list copy, doesn't make multiple passes through the list, doesn't require additional exception handling, and returns the matched object or None if there isn't a match. Only issue is that it makes for a long statement.
In general, when looking for a one-liner solution that doesn't throw exceptions, next() is the way to go, since it's one of the few Python functions that supports a default argument.
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?
Eek, don't do anything that complicated : )
Just filter()
your tags. bool()
returns False
for empty strings, so instead of
new_tag_list = f1.striplist(tag_string.split(",") + selected_tags)
you should write
new_tag_list = filter(bool, f1.striplist(tag_string.split(",") + selected_tags))
or better yet, put this logic inside striplist()
so that it doesn't return empty strings in the first place.
If index
doesn't find the searched string, it throws the ValueError
you're seeing. Either
catch the ValueError:
try:
i = s.index("")
del s[i]
except ValueError:
print "new_tag_list has no empty string"
or use find
, which returns -1 in that case.
i = s.find("")
if i >= 0:
del s[i]
else:
print "new_tag_list has no empty string"
Adding this answer for completeness, though it's only usable under certain conditions.
If you have very large lists, removing from the end of the list avoids CPython internals having to memmove
, for situations where you can re-order the list. It gives a performance gain to remove from the end of the list, since it won't need to memmove
every item after the one your removing - back one step (1).
For one-off removals the performance difference may be acceptable, but if you have a large list and need to remove many items - you will likely notice a performance hit.
Although admittedly, in these cases, doing a full list search is likely to be a performance bottleneck too, unless items are mostly at the front of the list.
This method can be used for more efficient removal,
as long as re-ordering the list is acceptable. (2)
def remove_unordered(ls, item):
i = ls.index(item)
ls[-1], ls[i] = ls[i], ls[-1]
ls.pop()
You may want to avoid raising an error when the item
isn't in the list.
def remove_unordered_test(ls, item):
try:
i = ls.index(item)
except ValueError:
return False
ls[-1], ls[i] = ls[i], ls[-1]
ls.pop()
return True
A simple way to test this, compare the speed difference from removing from the front of the list with removing the last element:
python -m timeit 'a = [0] * 100000' 'while a: a.remove(0)'
With:
python -m timeit 'a = [0] * 100000' 'while a: a.pop()'
(gives an order of magnitude speed difference where the second example is faster with CPython and PyPy).
set
, especially if the list isn't meant to store duplicates.set
. Also check on btree's if the data can be ordered.Source: Stackoverflow.com