Check if a given key already exists in a dictionary

Question

I wanted to test if a key exists in a dictionary before updating the value for the key  I wrote the following code   if  key1  in dict keys      print  blah  else    print  boo    I think this is not the best way to accomplish this task  Is there a better way to test for a key in the dictionary

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You don t have to call keys   if  key1  in dict    print  blah   else    print  boo     That will be much faster as it uses the dictionary s hashing as opposed to doing a linear search  which calling keys would do

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in is the intended way to test for the existence of a key in a dict   d     key1   10   key2   23   if  key1  in d      print  this will execute    if  nonexistent key  in d      print  this will not     If you wanted a default  you can always use dict get     d   dict    for i in range 100       key   i   10     d key    d get key  0    1   and if you wanted to always ensure a default value for any key you can either use dict setdefault   repeatedly or defaultdict from the collections module  like so   from collections import defaultdict  d   defaultdict int   for i in range 100       d i   10     1   but in general  the in keyword is the best way to do it

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Sharing one more way of checking if a key exists using boolean operators   d     a   1   b  2  keys    abcd   for k in keys      x    k in d and  blah   or  boo      print x     This returns   gt  gt  gt  blah  gt  gt  gt  blah  gt  gt  gt  boo  gt  gt  gt  boo   Explanation  First you should know that in Python  0  None  or objects with zero length evaluate to False  Everything else evaluates to True  Boolean operations are evaluated left to right and return the operand not True or False   Let s see an example    gt  gt  gt   Some string  or 1 0   Some string   gt  gt  gt    Since  Some string  evaluates to True  the rest of the or is not evaluated and there is no division by zero error raised   But if we switch the order 1 0 is evaluated first and raises an exception    gt  gt  gt  1 0 or  Some string  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  ZeroDivisionError  division by zero  gt  gt  gt     We can use this for pattern for checking if a key exists    k in d and  blah     does the same as  if k in d       blah  else      False   This already returns the correct result if the key exists  but we want it to print  boo  when it doesn t  So  we take the result and or it with  boo    gt  gt  gt  False or  boo   boo   gt  gt  gt   blah  or  boo   blah   gt  gt  gt

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For additional info on speed execution of the accepted answer s proposed methods  10m loops      key  in mydict elapsed time 1 07 sec mydict get  key   elapsed time 1 84 sec mydefaultdict  key   elapsed time 1 07 sec   Therefore using in or defaultdict are recommended against get

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Check if a given key already exists in a dictionary   To get the idea how to do that we first inspect what methods we can call on dictionary   Here are the methods    d   clear  0   copy  1   fromkeys  2   get  3   items  4   keys  5   pop  6   popitem  7   setdefault  8   update  9   values  10      Python Dictionary clear         Removes all Items Python Dictionary copy          Returns Shallow Copy of a Dictionary Python Dictionary fromkeys      Creates dictionary from given sequence Python Dictionary get           Returns Value of The Key Python Dictionary items         Returns view of dictionary  key  value  pair Python Dictionary keys          Returns View Object of All Keys Python Dictionary pop           Removes and returns element having given key Python Dictionary popitem       Returns  amp  Removes Element From Dictionary Python Dictionary setdefault    Inserts Key With a Value if Key is not Present Python Dictionary update        Updates the Dictionary  Python Dictionary values        Returns view of all values in dictionary     The brutal method to check if the key already exists may be the get   method   d get  key     The other two interesting methods items   and keys   sounds like too much of work  So let s examine if get   is the right method for us  We have our dict d   d    clear  0   copy  1   fromkeys  2   get  3   items  4   keys  5   pop  6   popitem  7   setdefault  8   update  9   values  10    Printing shows the key we don t have will return None   print d get  key     None print d get  clear     0 print d get  copy     1   We may use that to get the info if the key is present or no  But consider this if we create a dict with a single key None   d    key  None  print d get  key     None print d get  key2     None   Leading that get   method is not reliable in case some values may be None  This story should have a happier ending  If we use the in comparator   print  key  in d   True print  key2  in d   False   We get the correct results  We may examine the Python byte code   import dis dis dis   key  in d       1           0 LOAD CONST               0   key                   2 LOAD NAME                0  d                  4 COMPARE OP               6  in                  6 RETURN VALUE  dis dis  d get  key2         1           0 LOAD NAME                0  d                  2 LOAD METHOD              1  get                  4 LOAD CONST               0   key2                   6 CALL METHOD              1                 8 RETURN VALUE   This shows that in compare operator is not just more reliable but even faster than get

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You can use for loop to iterate over the dictionary and get the name of key you want to find in the dictionary  after that check if it exist or not using if condition    dic     first    12   second    123  for each in dic      if each     second            print  the key exists and the corresponding value can be updated in the dictionary

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What about using EAFP  easier to ask forgiveness than permission    try     blah   dict  mykey        key exists in dict except KeyError       key doesn t exist in dict   See other SO posts   Using try vs if in python or  Checking for member existence in Python

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Using ternary operator   message    blah  if  key1  in dict else  booh  print message

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Python dictionary has the method called   contains    This method will return True if the dictionary has the key else returns False     gt  gt  gt  temp         gt  gt  gt  help temp   contains     Help on built-in function   contains       contains   key     method of builtins dict instance     True if D has a key k  else False

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I would recommend using the setdefault method instead   It sounds like it will do everything you want    gt  gt  gt  d     foo   bar    gt  gt  gt  q   d setdefault  foo   baz    Do not override the existing key  gt  gt  gt  print q  The value takes what was originally in the dictionary bar  gt  gt  gt  print d   foo    bar    gt  gt  gt  r   d setdefault  baz  18   baz was never in the dictionary  gt  gt  gt  print r  Now r has the value supplied above 18  gt  gt  gt  print d  The dictionary s been updated   foo    bar    baz   18

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Dictionary in python has a get  key   default  method  So you can just set a default value in case there is no key    values         myValue   values get  Key   None

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You can test for the presence of a key in a dictionary  using the in keyword   d     a   1   b   2   a  in d    lt    evaluates to True  c  in d    lt    evaluates to False   A common use for checking the existence of a key in a dictionary before mutating it is to default-initialize the value  e g  if your values are lists  for example  and you want to ensure that there is an empty list to which you can append when inserting the first value for a key   In cases such as those  you may find the collections defaultdict   type to be of interest   In older code  you may also find some uses of has key    a deprecated method for checking the existence of keys in dictionaries  just use key name in dict name  instead

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Just an FYI adding to Chris  B  best answer    d   defaultdict int    Works as well  the reason is that calling int   returns 0 which is what defaultdict does behind the scenes  when constructing a dictionary   hence the name  Factory Function  in the documentation

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You can shorten this   if  key1  in dict            However  this is at best a cosmetic improvement  Why do you believe this is not the best way

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Python 2 only   and python 2 7 supports  in  already  you can use the has key   method  if dict has key  xyz    1       update the value for the key else      pass

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The ways in which you can get the results are    if your dict has key key  Removed in Python 3 if key in your dict try except block   Which is better is dependent on 3 things    Does the dictionary  normally has the key  or  normally does not have the key   Do you intend to use conditions like if   else   elseif   else  How big is dictionary    Read More  http   paltman com try-except-performance-in-python-a-simple-test   Use of try block instead of  in  or  if    try      my dict of items key i want to check  except KeyError        Do the operation you wanted to do for  key not present in dict   else        Do the operation you wanted to do with  key present in dict

[python] Check if a given key already exists in a dictionary

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