Python update a key in dict if it doesn t exist

Question

I want to insert a key-value pair into dict if key not in dict keys    Basically I could do it with   if key not in d keys        d key    value   But is there a better way  Or what s the pythonic solution to this problem

User · Answer

With the following you can insert multiple values and also have default values but you're creating a new dictionary.

d = {**{ key: value }, **default_values}

I've tested it with the most voted answer and on average this is faster as it can be seen in the following example, .

Speed test comparing a for loop based method with a dict comprehension with unpack operator method.

if no copy (d = default_vals.copy()) is made on the first case then the most voted answer would be faster once we reach orders of magnitude of 10**5 and greater. Memory footprint of both methods are the same.

User · Answer

Use dict setdefault      gt  gt  gt  d    1   one    gt  gt  gt  d setdefault 1   1    one   gt  gt  gt  d      d has not changed because the key already existed  1   one    gt  gt  gt  d setdefault 2   two    two   gt  gt  gt  d  1   one   2   two

User · Answer

Since Python 3 9 you can use the merge operator   to merge two dictionaries  The dict on the right takes precedence  new dict   old dict     key  val    For example  new dict      a   1   b   2        b   42    print new dict      a   1   b   42   Note  this creates a new dictionary with the updated values

User · Answer

According to the above answers setdefault   method worked for me  old attr name   mydict setdefault key  attr name  if attr name    old attr name      raise RuntimeError f quot Key   key   duplication   quot                         f quot   old attr name   and   attr name    quot    Though this solution is not generic  Just suited me in this certain case  The exact solution would be checking for the key first  as was already advised   but with setdefault   we avoid one extra lookup on the dictionary  that is  though small  but still a performance gain

User · Answer

You do not need to call d keys    so  if key not in d      d key    value   is enough  There is no clearer  more readable method   You could update again with dict get    which would return an existing value if the key is already present   d key    d get key  value    but I strongly recommend against this  this is code golfing  hindering maintenance and readability

[python] Python update a key in dict if it doesn't exist

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