Return None if Dictionary key is not available

Question

I need a way to get a dictionary value if its key exists  or simply return None  if it does not   However  Python raises a KeyError exception if you search for a key that does not exist  I know that I can check for the key  but I am looking for something more explicit  Is there a way to just return None if the key does not exist

User · Answer

You could use a dict object s get   method  as others have already suggested  Alternatively  depending on exactly what you re doing  you might be able use a try except suite like this   try      lt to do something with d key  gt  except KeyError      lt deal with it not being there gt    Which is considered to be a very  Pythonic  approach to handling the case

User · Answer

You should use the get   method from the dict class  d      r   d get  missing key   None    This will result in r    None   If the key isn t found in the dictionary  the get function returns the second argument

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You can use dict get    value   d get key    which will return None if key is not in d  You can also provide a different default value that will be returned instead of None   value   d get key   empty

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I usually use a defaultdict for situations like this  You supply a factory method that takes no arguments and creates a value when it sees a new key  It s more useful when you want to return something like an empty list on new keys  see the examples    from collections import defaultdict d   defaultdict lambda  None  print d  new key      prints  None

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If you want a more transparent solution  you can subclass dict to get this behavior   class NoneDict dict       def   getitem   self  key           return dict get self  key    gt  gt  gt  foo   NoneDict   1  asdf     2  qwerty      gt  gt  gt  foo 1   asdf   gt  gt  gt  foo 2   qwerty   gt  gt  gt  foo 3  is None True

User · Answer

As others have said above  you can use get     But to check for a key  you can also do   d      if  keyname  in d         d  keyname   exists     pass  else         d  keyname   does not exist     pass

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If you can do it with False  then  there s also the hasattr built-in funtion   e dict   hasattr e   message     gt  gt  gt  False

User · Answer

Wonder no more  It s built into the language            help dict       Help on class dict in module builtins       class dict object          dict   -  new empty dictionary         dict mapping  -  new dictionary initialized from a mapping object s              key  value  pairs                          get                  D get k  d   -  D k  if k in D  else d   d defaults to None

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I was thrown aback by what was possible in python2 vs python3  I will answer it based on what I ended up doing for python3  My objective was simple  check if a json response in dictionary format gave an error or not  My dictionary is called  token  and my key that I am looking for is  error   I am looking for key  error  and if it was not there setting it to value of None  then checking is the value is None  if so proceed with my code  An else statement would handle if I do have the key  error    if   token get  error   None   is None       do something

User · Answer

Use dict get  Returns the value for key if key is in the dictionary  else default  If default is not given  it defaults to None  so that this method never raises a KeyError

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For those using the dict get technique for nested dictionaries  instead of explicitly checking for every level of the dictionary  or extending the dict class  you can set the default return value to an empty dictionary except for the out-most level  Here s an example  my dict     level 1                   level 2                        level 3    more data                                                   result   my dict get  level 1       get  level 2       get  level 3     result - gt   more data  none result   my dict get  level 1       get  what level       get  level 3     none result - gt  None  WARNING  Please note that this technique only works if the expected key s value is a dictionary  If the key what level did exist in the dictionary but its value was a string or integer etc   then it would ve raised an AttributeError

User · Answer

A one line solution would be    item  key   if  key  in item else None   This is useful when trying to add dictionary values to a new list and want to provide a default    eg    row    item  key   if  key  in item else  default value

[python] Return None if Dictionary key is not available

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