Why doesn t list have safe get method like dictionary

Question

Why doesn t list have a safe  get  method like dictionary    gt  gt  gt  d     a   b    gt  gt  gt  d  a    b   gt  gt  gt  d  c   KeyError   c   gt  gt  gt  d get  c    fail    fail    gt  gt  gt  l    1   gt  gt  gt  l 10  IndexError  list index out of range

User · Answer

This guy worked for me  list get   lambda l  x  l x x 1  and l x  or 0  lambdas are great for one liner helper functions like this

User · Answer

For small index values you can implement my list get index  default  as  my list    default     index   1   index  If you know in advance what index is then this can be simplified  for example if you knew it was 1 then you could do  my list    default  default   index  Because lists are forward packed the only fail case we need to worry about is running off the end of the list  This approach pads the end of the list with enough defaults to guarantee that index is covered

User · Answer

Dictionaries are for look ups  It makes sense to ask if an entry exists or not   Lists are usually iterated   It isn t common to ask if L 10  exists but rather if the length of L is 11

User · Answer

This works if you want the first element  like my list get 0    gt  gt  gt  my list    1 2 3   gt  gt  gt  next iter my list    fail   1  gt  gt  gt  my list       gt  gt  gt  next iter my list    fail    fail    I know it s not exactly what you asked for but it might help others

User · Answer

A reasonable thing you can do is to convert the list into a dict and then access it with the get method   gt  gt  gt  my list     a    b    c    d    e    gt  gt  gt  my dict   dict enumerate my list    gt  gt  gt  print my dict  0   a   1   b   2   c   3   d   4   e    gt  gt  gt  my dict get 2   c   gt  gt  gt  my dict get 10   N A

User · Answer

Probably because it just didn t make much sense for list semantics  However  you can easily create your own by subclassing   class safelist list       def get self  index  default None           try              return self   getitem   index          except IndexError              return default  def  test        l   safelist range 10       print l get 20   oops    if   name         main          test

User · Answer

Try this    gt  gt  gt  i   3  gt  gt  gt  a    1  2  3  4   gt  gt  gt  next iter a i      fail   4  gt  gt  gt  next iter a i   1      fail    fail

User · Answer

So I did some more research into this and it turns out there isn t anything specific for this  I got excited when I found list index value   it returns the index of a specified item  but there isn t anything for getting the value at a specific index   So if you don t want to use the safe list get solution which I think is pretty good  Here are some 1 liner if statements that can get the job done for you depending on the scenario    gt  gt  gt  x    1  2  3   gt  gt  gt  el   x 4  if len x   gt  4 else  No   gt  gt  gt  el  No    You can also use None instead of  No   which makes more sense     gt  gt  gt  x    1  2  3   gt  gt  gt  i   2  gt  gt  gt  el i   x i  if len x     i 1 else None   Also if you want to just get the first or last item in the list  this works  end el   x -1  if x else None   You can also make these into functions but I still liked the IndexError exception solution  I experimented with a dummied down version of the safe list get solution and made it a bit simpler  no default    def list get l  i       try          return l i      except IndexError          return None   Haven t benchmarked to see what is fastest

User · Answer

Your usecase is basically only relevant for when doing arrays and matrixes of a fixed length  so that you know how long they are before hand  In that case you typically also create them before hand filling them up with None or 0  so that in fact any index you will use already exists   You could say this  I need  get   on dictionaries quite often  After ten years as a full time programmer I don t think I have ever needed it on a list

User · Answer

Ultimately it probably doesn t have a safe  get method because a dict is an associative collection  values are associated with names  where it is inefficient to check if a key is present  and return its value  without throwing an exception  while it is super trivial to avoid exceptions accessing list elements  as the len method is very fast    The  get method allows you to query the value associated with a name  not directly access the 37th item in the dictionary  which would be more like what you re asking of your list    Of course  you can easily implement this yourself   def safe list get  l  idx  default     try      return l idx    except IndexError      return default   You could even monkeypatch it onto the   builtins   list constructor in   main    but that would be a less pervasive change since most code doesn t use it   If you just wanted to use this with lists created by your own code you could simply subclass list and add the get method

User · Answer

Credits to jose angel jimenez and Gus Bus   For the  quot oneliner quot  fans     If you want the first element of a list or if you want a default value if the list is empty try  liste     a    b    c   value    liste 0 1  or   default     0  print value   returns a and liste      value    liste 0 1  or   default     0  print value   returns default  Examples for other elements    liste     a    b    c   print liste 0 1      returns   a   print liste 1 2      returns   b   print liste 2 3      returns   c   print liste 3 4      returns     With default fallback    liste     a    b    c   print  liste 0 1  or   default     0      returns a print  liste 1 2  or   default     0      returns b print  liste 2 3  or   default     0      returns c print  liste 3 4  or   default     0      returns default   Possibly shorter  liste     a    b    c   value    liste  1  or   default    print value     returns a  It looks like you need the comma before the equal sign  the equal sign and the latter parenthesis   More general  liste     a    b    c   f   lambda l  x  d  l x x 1  and l x  or d print f liste  0   default       returns a print f liste  1   default       returns b print f liste  2   default       returns c print f liste  3   default       returns default   Tested with Python 3 6 0  v3 6 0 41df79263a11  Dec 22 2016  17 23 13

User · Answer

Instead of using  get  using like this should be ok for lists  Just a usage difference    gt  gt  gt  l    1   gt  gt  gt  l 10  if 10  lt  len l  else  fail   fail

[python] Why doesn't list have safe "get" method like dictionary?

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