Best way to handle list index might-not-exist in python

Question

I have code which looks something like this   thing index   thing list index thing  otherfunction thing list  thing index    ok so that s simplified but you get the idea  Now thing might not actually be in the list  in which case I want to pass -1 as thing index  In other languages this is what you d expect index   to return if it couldn t find the element  In fact it throws a ValueError   I could do this   try      thing index   thing list index thing  except ValueError      thing index   -1 otherfunction thing list  thing index    But this feels dirty  plus I don t know if ValueError could be raised for some other reason  I came up with the following solution based on generator functions  but it seems a little complex   thing index       i for i in xrange len thing list   if thing list i   thing   or  -1    0    Is there a cleaner way to achieve the same thing  Let s assume the list isn t sorted

User · Answer

There is nothing  dirty  about using try-except clause  This is the pythonic way  ValueError will be raised by the  index method only  because it s the only code you have there   To answer the comment  In Python  easier to ask forgiveness than to get permission philosophy is well established  and no index will not raise this type of error for any other issues  Not that I can think of any

User · Answer

There is nothing wrong with your code that uses ValueError  Here s yet another one-liner if you d like to avoid exceptions   thing index   next  i for i  x in enumerate thing list  if x    thing   -1

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I d suggest   if thing in thing list    list index   -1 else    list index   thing list index thing

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What about this   otherfunction thing collection  thing    Rather than expose something so implementation-dependent like a list index in a function interface  pass the collection and the thing and let otherfunction deal with the  test for membership  issues   If otherfunction is written to be collection-type-agnostic  then it would probably start with   if thing in thing collection          proceed with operation on thing   which will work if thing collection is a list  tuple  set  or dict   This is possibly clearer than   if thing index    MAGIC VALUE INDICATING NOT A MEMBER    which is the code you already have in otherfunction

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I don t know why you should think it is dirty    because of the exception  if you want a oneliner  here it is   thing index   thing list index elem  if thing list count elem  else -1   but i would advise against using it  I think Ross Rogers solution is the best  use an object to encapsulate your desiderd behaviour  don t try pushing the language to its limits at the cost of readability

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It s been quite some time but it s a core part of the stdlib and has dozens of potential methods so I think it s useful to have some benchmarks for the different suggestions and include the numpy method which can be by far the fastest  import random from timeit import timeit import numpy as np  l    random random   for i in range 10  4   l 10  4 - 100    5    method 1 def fun1 l list  x int  e   -1  - gt  int      return   i for i elem in enumerate l  if elem    x  or  e   0     method 2 def fun2 l list  x int  e   -1  - gt  int      for i elem in enumerate l           if elem    x              return i     else          return e    method 3 def fun3 l list  x int  e   -1  - gt  int      try          idx   l index x      except ValueError          idx   e     return idx    method 4 def fun4 l list  x int  e   -1  - gt  int      return l index x  if x in l else e  l2   np array l    method 5 def fun5 l list or np ndarray  x int  e   -1  - gt  int      res   np where np equal l  x       if res 0  any            return res 0  0      else                  return e   if   name       quot   main   quot       print  quot Method 1  quot       print timeit stmt    quot fun1 l  5  quot   number   1000  globals   globals         print  quot  quot       print  quot Method 2  quot       print timeit stmt    quot fun2 l  5  quot   number   1000  globals   globals         print  quot  quot       print  quot Method 3  quot       print timeit stmt    quot fun3 l  5  quot   number   1000  globals   globals         print  quot  quot       print  quot Method 4  quot       print timeit stmt    quot fun4 l  5  quot   number   1000  globals   globals         print  quot  quot       print  quot Method 5  numpy given list  quot       print timeit stmt    quot fun5 l  5  quot   number   1000  globals   globals         print  quot  quot       print  quot Method 6  numpy given np ndarray  quot       print timeit stmt    quot fun5 l2  5  quot   number   1000  globals   globals         print  quot  quot    When run as main  this results in the following printout on my machine indicating time in seconds to complete 1000 trials of each function  Method 1  0 7502102799990098 Method 2  0 7291318440002215 Method 3  0 24142152300009911 Method 4  0 5253471979995084 Method 5  numpy given list  0 5045417560013448 Method 6  numpy given np ndarray  0 011147511999297421 Of course the question asks specifically about lists so the best solution is to use the try-except method  however the speed improvements  at least 20x here compared to try-except  offered by using the numpy data structures and operators instead of python data structures is significant and if building something on many arrays of data that is performance critical then the author should try to use numpy throughout to take advantage of the superfast C bindings   CPython interpreter  other interpreter performances may vary  Btw  the reason Method 5 is much slower than Method 6 is because numpy first has to convert the given list to it s own numpy array  so giving it a list doesn t break it it just doesn t fully utilise the speed possible

User · Answer

I have the same issue with the   index    method on lists  I have no issue with the fact that it throws an exception but I strongly disagree with the fact that it s a non-descriptive ValueError  I could understand if it would ve been an IndexError  though   I can see why returning  -1  would be an issue too because it s a valid index in Python  But realistically  I never expect a   index    method to return a negative number   Here goes a one liner  ok  it s a rather long line       goes through the list exactly once and returns  None  if the item isn t found  It would be trivial to rewrite it to return -1  should you so desire   indexOf   lambda list  thing                reduce lambda acc   idx  elem                        idx if  acc is None  and elem    thing else acc  list  None    How to use    gt  gt  gt  indexOf  1 2 3   4   gt  gt  gt   gt  gt  gt  indexOf  1 2 3   1  0  gt  gt  gt

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Comparison of implementations Simple comparison on Python 3 8 TL DR maybeidx2 is faster in general except for arrays  n lt 100  with lots of misses def maybeidx1 l  v       return l index v  if v in l else None  def maybeidx2 l  v       try          return l index v      except ValueError          return None  Test cases  a     range 100 000     Case 1  index in list maybeidx1 a  50 000  Out 20   50000 maybeidx2 a  50 000  Out 21   50000   Case 2  index not in list maybeidx1 a  100 000  is None Out 23   True maybeidx2 a  100 000  is None Out 24   True  Timing case 1  timeit maybeidx1 a  50 000  1 06 ms    15 6   s per loop  mean    std  dev  of 7 runs  1000 loops each   timeit maybeidx2 a  50 000  530   s    8 47   s per loop  mean    std  dev  of 7 runs  1000 loops each   Timing case 2  timeit maybeidx1 a  100 000  1 07 ms    21   s per loop  mean    std  dev  of 7 runs  1000 loops each   timeit maybeidx2 a  100 000  1 07 ms    16 6   s per loop  mean    std  dev  of 7 runs  1000 loops each   Result Use the maybeidx2 method for larger arrays  This is faster due to maybeidx1 having two scans of the array in search of the value - this is still O n  time but has a constant multiplier of 2 and thus slower in practice  This holds in the case of the value being present in the list  When the value is not present  these times will be roughly equal  they both have to scan the full array exactly once  then return None  The overhead of the try-except is negligible  even with an array size of 10 - unless case two occurs  Then the try-except overhead is noticeable  Example  a     range 10    timeit maybeidx1 a  10  191 ns    2 61 ns per loop  mean    std  dev  of 7 runs  10000000 loops each   timeit maybeidx2 a  10  566 ns    5 93 ns per loop  mean    std  dev  of 7 runs  1000000 loops each   This overhead becomes negligible  on my machine  when a has above 100 elements

User · Answer

This issue is one of language philosophy  In Java for example there has always been a tradition that exceptions should really only be used in  exceptional circumstances  that is when errors have happened  rather than for flow control  In the beginning this was for performance reasons as Java exceptions were slow but now this has become the accepted style   In contrast Python has always used exceptions to indicate normal program flow  like raising a ValueError as we are discussing here  There is nothing  dirty  about this in Python style and there are many more where that came from  An even more common example is StopIteration exception which is raised by an iterator   s next   method to signal that there are no further values

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What about this      li    1 2 3 4 5    create list   li   dict zip li range len li       convert List To Dict  print  li      1  0  2  1  3  2  4 3   5  4  li get 20    None  li get 1     0

User · Answer

What about like this   temp inx    L    x   index x   inx   temp inx if temp inx  lt  len L  else -1

User · Answer

thing index   thing list index elem  if elem in thing list else -1   One line  Simple  No exceptions

User · Answer

If you are doing this often then it is better to stove it away in a helper function   def index of val  in list       try          return in list index val      except ValueError          return -1

User · Answer

The dict type has a get function  where if the key doesn t exist in the dictionary  the 2nd argument to get is the value that it should return   Similarly there is setdefault  which returns the value in the dict if the key exists  otherwise it sets the value according to your default parameter and then returns your default parameter   You could extend the list type to have a getindexdefault method   class SuperDuperList list       def getindexdefault self  elem  default           try              thing index   self index elem              return thing index         except ValueError              return default   Which could then be used like   mylist   SuperDuperList  0 1 2   index   mylist getindexdefault   asdf   -1

[python] Best way to handle list.index(might-not-exist) in python?

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