Determine whether a key is present in a dictionary

Question

Possible Duplicate      39 has key    39  or   39 in  39         I have a Python dictionary like    mydict     name   abc   city   xyz   country   def     I want to check if a key is in dictionary or not  I am eager to know that which is more preferable from the following two cases and why   1 gt  if mydict has key  name    2 gt  if  name  in mydict

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In the same vein as martineau s response  the best solution is often not to check  For example  the code  if x in d      foo   d x  else      foo   bar   is normally written   foo   d get x  bar    which is shorter and more directly speaks to what you mean   Another common case is something like  if x not in d      d x        d x  append foo    which can be rewritten  d setdefault x      append foo    or rewritten even better by using a collections defaultdict list  for d and writing  d x  append foo

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My answer is  neither one    I believe the most  Pythonic  way to do things is to NOT check beforehand if the key is in a dictionary and instead just write code that assumes it s there and catch any KeyErrors that get raised because it wasn t     This is usually done with enclosing the code in a try   except clause and is a well-known idiom usually expressed as  It s easier to ask forgiveness than permission  or with the acronym EAFP  which basically means it is better to try something and catch the errors instead for making sure everything s OK before doing anything  Why validate what doesn t need to be validated when you can handle exceptions gracefully instead of trying to avoid them  Because it s often more readable and the code tends to be faster if the probability is low that the key won t be there  or whatever preconditions there may be    Of course  this isn t appropriate in all situations and not everyone agrees with the philosophy  so you ll need to decide for yourself on a case-by-case basis  Not surprisingly the opposite of this is called LBYL for  Look Before You Leap    As a trivial example consider   if  name  in dct      value   dct  name     3 else      logerror    s  not found in dictionary  using default    name      value   42   vs  try      value   dct  name     3 except KeyError      logerror    s  not found in dictionary  using default    name      value   42   Although in the case it s almost exactly the same amount of code  the second doesn t spend time checking first and is probably slightly faster because of it  try   except block isn t totally free though  so it probably doesn t make that much difference here    Generally speaking  testing in advance can often be much more involved and the savings gain from not doing it can be significant  That said  if  name  in dict  is better for the reasons stated in the other answers   If you re interested in the topic  this message titled  EAFP vs LBYL  was Re  A little disappointed so far   from the Python mailing list archive probably explains the difference between the two approached better than I have here  There s also a good discussion about the two approaches in the book Python in a Nutshell  2nd Ed by Alex Martelli in chapter 6 on Exceptions titled Error-Checking Strategies   I see there s now a newer 3rd edition  publish in 2017  which covers both Python 2 7 and 3 x

User · Answer

In terms of bytecode  in saves a LOAD ATTR and replaces a CALL FUNCTION with a COMPARE OP    gt  gt  gt  dis dis indict    2           0 LOAD GLOBAL              0  name                3 LOAD GLOBAL              1  d                6 COMPARE OP               6  in                9 POP TOP                 gt  gt  gt  dis dis haskey    2           0 LOAD GLOBAL              0  d                3 LOAD ATTR                1  haskey                6 LOAD GLOBAL              2  name                9 CALL FUNCTION            1              12 POP TOP                My feelings are that in is much more readable and is to be preferred in every case that I can think of   In terms of performance  the timing reflects the opcode    python -mtimeit -s d   dict  i  i  for i in range 10000      foo  in d   10000000 loops  best of 3  0 11 usec per loop    python -mtimeit -s d   dict  i  i  for i in range 10000     d has key  foo      1000000 loops  best of 3  0 205 usec per loop   in is almost twice as fast

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if  name  in mydict    is the preferred  pythonic version  Use of has key   is discouraged  and this method has been removed in Python 3

[python] Determine whether a key is present in a dictionary

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