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    gt  gt  gt  s    u    u Hello   u Cool   u Glam    gt  gt  gt  i   s index      gt  gt  gt  del s i   gt  gt  gt  s  u Hello   u Cool   u Glam     But if there is no empty string    gt  gt  gt  s    u Hello   u Cool   u Glam    gt  gt  gt  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   lt pyshell 30 gt    line 1  in  lt module gt      if new tag list index              ValueError  list index x   x not in list   Why does this happen  and how do I work around it

User · 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     See this Python history blog post by GvR  This syntax is inspired by the set-builder notation in math  Python 3 has also set and dict comprehensions    Notes   you may want to use the inequality operator    instead of is not  the difference is important  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

User · Answer

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

User · Answer

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  gt   0      del s i  else      print  new tag list has no empty string

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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      While I tested this with CPython  its quite likely most all other Python implementations use an array to store lists internally  So unless they use a sophisticated data structure designed for efficient list re-sizing  they likely have the same performance characteristic       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      In this case you might consider using a set  especially if the list isn t meant to store duplicates In practice though you may need to store mutable data which can t be added to a set  Also check on btree s if the data can be ordered

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

1) Almost-English style:

2) Duck-typed, EAFP style:

3) Functional style:

4) Mathematical style:

Notes

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