Check if a given key already exists in a dictionary and increment it

Question

Given a dictionary  how can I find out if a given key in that dictionary has already been set to a non-None value   I e   I want to do this   my dict       if  my dict key     None     my dict key    1 else    my dict key     1   I e   I want to increment the value if there s already one there  or set it to 1 otherwise

User · Answer

You need the key in dict idiom for that   if key in my dict and not  my dict key  is None       do something else      do something else   However  you should probably consider using defaultdict  as dF suggested

User · Answer

As you can see from the many answers  there are several solutions  One instance of LBYL  look before you leap  has not been mentioned yet  the has key   method   my dict       def add  key       if my dict has key key           my dict key     1     else          my dict key    1  if   name         main         add  foo       add  bar       add  foo       print my dict

User · Answer

The way you are trying to do it is called LBYL  look before you leap   since you are checking conditions before trying to increment your value   The other approach is called EAFP  easier to ask forgiveness then permission    In that case  you would just try the operation  increment the value    If it fails  you catch the exception and set the value to 1   This is a slightly more Pythonic way to do it  IMO    http   mail python org pipermail python-list 2003-May 205182 html

User · Answer

You are looking for collections defaultdict  available for Python 2 5    This  from collections import defaultdict  my dict   defaultdict int  my dict key     1   will do what you want   For regular Python dicts  if there is no value for a given key  you will not get None when accessing the dict -- a KeyError will be raised  So if you want to use a regular dict  instead of your code you would use  if key in my dict      my dict key     1 else      my dict key    1

User · Answer

As you can see from the many answers  there are several solutions  One instance of LBYL  look before you leap  has not been mentioned yet  the has key   method   my dict       def add  key       if my dict has key key           my dict key     1     else          my dict key    1  if   name         main         add  foo       add  bar       add  foo       print my dict

User · Answer

I personally like using setdefault    my dict       my dict setdefault some key  0  my dict some key     1

User · Answer

The way you are trying to do it is called LBYL  look before you leap   since you are checking conditions before trying to increment your value   The other approach is called EAFP  easier to ask forgiveness then permission    In that case  you would just try the operation  increment the value    If it fails  you catch the exception and set the value to 1   This is a slightly more Pythonic way to do it  IMO    http   mail python org pipermail python-list 2003-May 205182 html

User · Answer

I prefer to do this in one line of code    my dict       my dict some key    my dict get some key  0    1   Dictionaries have a function  get  which takes two parameters - the key you want  and a default value if it doesn t exist  I prefer this method to defaultdict as you only want to handle the case where the key doesn t exist in this one line of code  not everywhere

User · Answer

I was looking for it  didn t found it on web then tried my luck with Try Error and found it  my dict       if my dict   contains   some key     my dict some key     1 else    my dict some key    1

User · Answer

I prefer to do this in one line of code    my dict       my dict some key    my dict get some key  0    1   Dictionaries have a function  get  which takes two parameters - the key you want  and a default value if it doesn t exist  I prefer this method to defaultdict as you only want to handle the case where the key doesn t exist in this one line of code  not everywhere

User · Answer

Here s one-liner that I came up with recently for solving this problem  It s based on the setdefault dictionary method   my dict      my dict key    my dict setdefault key  0    1

User · Answer

Agreed with cgoldberg  How I do it is   try      dict key     1 except KeyError      dict key    1   So either do it as above  or use a default dict as others have suggested  Don t use if statements  That s not Pythonic

User · Answer

Here s one-liner that I came up with recently for solving this problem  It s based on the setdefault dictionary method   my dict      my dict key    my dict setdefault key  0    1

User · Answer

To answer the question  how can I find out if a given index in that dict has already been set to a non-None value   I would prefer this   try    nonNone   my dict key  is not None except KeyError    nonNone   False   This conforms to the already invoked concept of EAFP  easier to ask forgiveness then permission   It also avoids the duplicate key lookup in the dictionary as it would in key in my dict and my dict key  is not None what is interesting if lookup is expensive   For the actual problem that you have posed  i e  incrementing an int if it exists  or setting it to a default value otherwise  I also recommend the  my dict key    my dict get key  default    1   as in the answer of Andrew Wilkinson   There is a third solution if you are storing modifyable objects in your dictionary  A common example for this is a multimap  where you store a list of elements for your keys  In that case  you can use   my dict setdefault key      append item    If a value for key does not exist in the dictionary  the setdefault method will set it to the second parameter of setdefault  It behaves just like a standard my dict key   returning the value for the key  which may be the newly set value

User · Answer

I prefer to do this in one line of code    my dict       my dict some key    my dict get some key  0    1   Dictionaries have a function  get  which takes two parameters - the key you want  and a default value if it doesn t exist  I prefer this method to defaultdict as you only want to handle the case where the key doesn t exist in this one line of code  not everywhere

User · Answer

You need the key in dict idiom for that   if key in my dict and not  my dict key  is None       do something else      do something else   However  you should probably consider using defaultdict  as dF suggested

User · Answer

I was looking for it  didn t found it on web then tried my luck with Try Error and found it  my dict       if my dict   contains   some key     my dict some key     1 else    my dict some key    1

User · Answer

The way you are trying to do it is called LBYL  look before you leap   since you are checking conditions before trying to increment your value   The other approach is called EAFP  easier to ask forgiveness then permission    In that case  you would just try the operation  increment the value    If it fails  you catch the exception and set the value to 1   This is a slightly more Pythonic way to do it  IMO    http   mail python org pipermail python-list 2003-May 205182 html

User · Answer

The way you are trying to do it is called LBYL  look before you leap   since you are checking conditions before trying to increment your value   The other approach is called EAFP  easier to ask forgiveness then permission    In that case  you would just try the operation  increment the value    If it fails  you catch the exception and set the value to 1   This is a slightly more Pythonic way to do it  IMO    http   mail python org pipermail python-list 2003-May 205182 html

User · Answer

I prefer to do this in one line of code    my dict       my dict some key    my dict get some key  0    1   Dictionaries have a function  get  which takes two parameters - the key you want  and a default value if it doesn t exist  I prefer this method to defaultdict as you only want to handle the case where the key doesn t exist in this one line of code  not everywhere

User · Answer

You are looking for collections defaultdict  available for Python 2 5    This  from collections import defaultdict  my dict   defaultdict int  my dict key     1   will do what you want   For regular Python dicts  if there is no value for a given key  you will not get None when accessing the dict -- a KeyError will be raised  So if you want to use a regular dict  instead of your code you would use  if key in my dict      my dict key     1 else      my dict key    1

User · Answer

Agreed with cgoldberg  How I do it is   try      dict key     1 except KeyError      dict key    1   So either do it as above  or use a default dict as others have suggested  Don t use if statements  That s not Pythonic

User · Answer

This isn t directly answering the question  but to me  it looks like you might want the functionality of collections Counter   from collections import Counter  to count     foo    foo    bar    baz    foo    bar    count   Counter to count   print count   print  acts just like the desired dictionary    print  bar occurs    times  format count  bar      print  any item that does not occur in the list is set to 0    print  dog occurs    times  format count  dog      print  can iterate over items from most frequent to least    for item  times in count most common        print     occurs    times  format item  times     This results in the output  Counter   foo   3   bar   2   baz   1   acts just like the desired dictionary  bar occurs 2 times any item that does not occur in the list is set to 0  dog occurs 0 times can iterate over items from most frequent to least  foo occurs 3 times bar occurs 2 times baz occurs 1 times

User · Answer

You are looking for collections defaultdict  available for Python 2 5    This  from collections import defaultdict  my dict   defaultdict int  my dict key     1   will do what you want   For regular Python dicts  if there is no value for a given key  you will not get None when accessing the dict -- a KeyError will be raised  So if you want to use a regular dict  instead of your code you would use  if key in my dict      my dict key     1 else      my dict key    1

User · Answer

You need the key in dict idiom for that   if key in my dict and not  my dict key  is None       do something else      do something else   However  you should probably consider using defaultdict  as dF suggested

User · Answer

A bit late but this should work   my dict      my dict key    my dict key    1 if key in my dict else 1

User · Answer

Agreed with cgoldberg  How I do it is   try      dict key     1 except KeyError      dict key    1   So either do it as above  or use a default dict as others have suggested  Don t use if statements  That s not Pythonic

User · Answer

As you can see from the many answers  there are several solutions  One instance of LBYL  look before you leap  has not been mentioned yet  the has key   method   my dict       def add  key       if my dict has key key           my dict key     1     else          my dict key    1  if   name         main         add  foo       add  bar       add  foo       print my dict

User · Answer

To answer the question  how can I find out if a given index in that dict has already been set to a non-None value   I would prefer this   try    nonNone   my dict key  is not None except KeyError    nonNone   False   This conforms to the already invoked concept of EAFP  easier to ask forgiveness then permission   It also avoids the duplicate key lookup in the dictionary as it would in key in my dict and my dict key  is not None what is interesting if lookup is expensive   For the actual problem that you have posed  i e  incrementing an int if it exists  or setting it to a default value otherwise  I also recommend the  my dict key    my dict get key  default    1   as in the answer of Andrew Wilkinson   There is a third solution if you are storing modifyable objects in your dictionary  A common example for this is a multimap  where you store a list of elements for your keys  In that case  you can use   my dict setdefault key      append item    If a value for key does not exist in the dictionary  the setdefault method will set it to the second parameter of setdefault  It behaves just like a standard my dict key   returning the value for the key  which may be the newly set value

User · Answer

Agreed with cgoldberg  How I do it is   try      dict key     1 except KeyError      dict key    1   So either do it as above  or use a default dict as others have suggested  Don t use if statements  That s not Pythonic

User · Answer

You are looking for collections defaultdict  available for Python 2 5    This  from collections import defaultdict  my dict   defaultdict int  my dict key     1   will do what you want   For regular Python dicts  if there is no value for a given key  you will not get None when accessing the dict -- a KeyError will be raised  So if you want to use a regular dict  instead of your code you would use  if key in my dict      my dict key     1 else      my dict key    1

User · Answer

As you can see from the many answers  there are several solutions  One instance of LBYL  look before you leap  has not been mentioned yet  the has key   method   my dict       def add  key       if my dict has key key           my dict key     1     else          my dict key    1  if   name         main         add  foo       add  bar       add  foo       print my dict

User · Answer

A bit late but this should work   my dict      my dict key    my dict key    1 if key in my dict else 1

User · Answer

You need the key in dict idiom for that   if key in my dict and not  my dict key  is None       do something else      do something else   However  you should probably consider using defaultdict  as dF suggested

User · Answer

This isn t directly answering the question  but to me  it looks like you might want the functionality of collections Counter   from collections import Counter  to count     foo    foo    bar    baz    foo    bar    count   Counter to count   print count   print  acts just like the desired dictionary    print  bar occurs    times  format count  bar      print  any item that does not occur in the list is set to 0    print  dog occurs    times  format count  dog      print  can iterate over items from most frequent to least    for item  times in count most common        print     occurs    times  format item  times     This results in the output  Counter   foo   3   bar   2   baz   1   acts just like the desired dictionary  bar occurs 2 times any item that does not occur in the list is set to 0  dog occurs 0 times can iterate over items from most frequent to least  foo occurs 3 times bar occurs 2 times baz occurs 1 times

User · Answer

I personally like using setdefault    my dict       my dict setdefault some key  0  my dict some key     1

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

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