How To Check If A Key in kwargs Exists

Question

Python 3 2 3  There were some ideas listed here  which work on regular var s  but it seems   kwargs play by different rules    so why doesn t this work and how can I check to see if a key in   kwargs exists   if kwargs  errormessage        print  It exists     I also think this should work  but it doesn t --  if errormessage in kwargs      print  yeah it s here     I m guessing because kwargs is iterable  Do I have to iterate through it just to check if a particular key is there

User · Answer

It is just this   if  errormessage  in kwargs      print  yeah it s here     You need to check  if the key is in the dictionary  The syntax for that is some key in some dict  where some key is something hashable  not necessarily a string    The ideas you have linked  these ideas  contained examples for checking if specific key existed in dictionaries returned by locals   and globals    Your example is similar  because you are checking existence of specific key in kwargs dictionary  the dictionary containing keyword arguments

User · Answer

You can discover those things easily by yourself   def hello  args    kwargs       print kwargs     print type kwargs      print dir kwargs   hello what  world

User · Answer

if kwarg   len        0      print kwarg

User · Answer

DSM s and Tadeck s answers answer your question directly   In my scripts I often use the convenient dict pop   to deal with optional  and additional arguments  Here s an example of a simple print   wrapper   def my print  args    kwargs       prefix   kwargs pop  prefix           print prefix   args    kwargs    Then    gt  gt  gt  my print  eggs    eggs  gt  gt  gt  my print  eggs   prefix  spam   spam eggs   As you can see  if prefix is not contained in kwargs  then the default     empty string  is being stored in the local prefix variable  If it is given  then its value is being used   This is generally a compact and readable recipe for writing wrappers for any kind of function  Always just pass-through arguments you don t understand  and don t even know if they exist  If you always pass through  args and   kwargs you make your code slower  and requires a bit more typing  but if interfaces of the called function  in this case print  changes  you don t need to change your code  This approach reduces development time while supporting all interface changes

User · Answer

You want  if  errormessage  in kwargs      print  found it     To get the value of errormessage  if  errormessage  in kwargs      print  errormessage equals     kwargs get  errormessage      In this way  kwargs is just another dict   Your first example  if kwargs  errormessage    means  get the value associated with the key  errormessage  in kwargs  and then check its bool value    So if there s no such key  you ll get a KeyError   Your second example  if errormessage in kwargs   means  if kwargs contains the element named by  errormessage   and unless  errormessage  is the name of a variable  you ll get a NameError   I should mention that dictionaries also have a method  get   which accepts a default parameter  itself defaulting to None   so that kwargs get  errormessage   returns the value if that key exists and None otherwise  similarly kwargs get  errormessage   17  does what you might think it does    When you don t care about the difference between the key existing and having None as a value or the key not existing  this can be handy

User · Answer

One way is to add it by yourself   How   By merging kwargs with a bunch of defaults  This won t be appropriate on all occasions  for example  if the keys are not known to you in advance  However  if they are  here is a simple example   import sys  def myfunc   kwargs       args     country   England   town   London                currency   Pound    language   English        diff   set kwargs keys    - set args keys        if diff          print  Invalid args   tuple diff  file sys stderr          return      args update kwargs                  print args    The defaults are set in the dictionary args  which includes all the keys we are expecting   We first check to see if there are any unexpected keys in kwargs   Then we update args with kwargs which will overwrite any new values that the user has set   We don t need to test if a key exists  we now use args as our argument dictionary and have no further need of kwargs

[python] How To Check If A Key in **kwargs Exists?

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