How can I find the number of arguments of a Python function

Question

How can I find the number of arguments of a Python function  I need to know how many normal arguments it has and how many named arguments   Example   def someMethod self  arg1  kwarg1 None       pass   This method has 2 arguments and 1 named argument

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Adding to the above  I ve also seen that the most of the times help   function really helps  For eg  it gives all the details about the arguments it takes    help  lt method gt     gives the below  method self    kwargs  method of apiclient discovery Resource instance Retrieves a report which is a collection of properties   statistics for a specific customer   Args    date  string  Represents the date in yyyy-mm-dd format for which the data is to be fetched   required    pageToken  string  Token to specify next page    parameters  string  Represents the application name  parameter name pairs to fetch in csv as app name1 param name1  app name2 param name2   Returns    An object of the form           JSON template for a collection of usage reports       nextPageToken    A String     Token for retrieving the next page      kind    admin reports usageReports     Th

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inspect getargspec   to meet your needs  from inspect import getargspec  def func a  b       pass print len getargspec func  args

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In   import inspect   class X      def xyz self  a  b  c            return   print len inspect getfullargspec X xyz  args     Out      4     Note  If xyz wasn t inside class X and had no  self  and just  a  b  c   then it would have printed 3    For python below 3 5  you may want to replace inspect getfullargspec by inspect getargspec in the code above

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As other answers suggest  getargspec works well as long as the thing being queried is actually a function  It does not work for built-in functions such as open  len  etc  and will throw an exception in such cases   TypeError   lt built-in function open gt  is not a Python function   The below function  inspired by this answer  demonstrates a workaround  It returns the number of args expected by f   from inspect import isfunction  getargspec def num args f     if isfunction f       return len getargspec f  args    else      spec   f   doc   split   n   0      args   spec spec find      1 spec find           return args count      1 if args else 0   The idea is to parse the function spec out of the   doc   string  Obviously this relies on the format of said string so is hardly robust

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inspect getargspec    Get the names and default values of a function   s arguments  A tuple of four things is returned   args  varargs  varkw  defaults   args is a list of the argument names  it may contain nested lists   varargs and varkw are the names of the   and    arguments or None  defaults is a tuple of default argument values or None if there are no default arguments  if this tuple has n elements  they correspond to the last n elements listed in args  Changed in version 2 6  Returns a named tuple ArgSpec args  varargs  keywords  defaults    See can-you-list-the-keyword-arguments-a-python-function-receives

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Good news for folks who want to do this in a portable way between Python 2 and Python 3 6   use inspect getfullargspec   method  It works in both Python 2 x and 3 6       As Jim Fasarakis Hilliard and others have pointed out  it used to be like this  1  In Python 2 x  use inspect getargspec   2  In Python 3 x  use signature  as getargspec   and getfullargspec   were deprecated     However  starting Python 3 6  by popular demand    things have changed towards better      From the Python 3 documentation page      inspect getfullargspec func       Changed in version 3 6  This method was previously documented as deprecated in favour of signature   in Python 3 5  but that decision has been reversed in order to restore a clearly supported standard interface for single-source Python 2 3 code migrating away from the legacy getargspec   API

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The previously accepted answer has been deprecated as of Python 3 0  Instead of using inspect getargspec you should now opt for the Signature class which superseded it    Creating a Signature for the function is easy via the signature function   from inspect import signature  def someMethod self  arg1  kwarg1 None       pass  sig   signature someMethod    Now  you can either view its parameters quickly by string it   str sig     returns    self  arg1  kwarg1 None     or you can also get a mapping of attribute names to parameter objects via sig parameters    params   sig parameters  print params  kwarg1      prints  kwarg1 20   Additionally  you can call len on sig parameters to also see the number of arguments this function requires   print len params      3   Each entry in the params mapping is actually a Parameter object that has further attributes making your life easier  For example  grabbing a parameter and viewing its default value is now easily performed with   kwarg1   params  kwarg1   kwarg1 default   returns  None   similarly for the rest of the objects contained in parameters     As for Python 2 x users  while inspect getargspec isn t deprecated  the language will soon be  -   The Signature class isn t available in the 2 x series and won t be  So you still need to work with inspect getargspec   As for transitioning between Python 2 and 3  if you have code that relies on the interface of getargspec in Python 2 and switching to signature in 3 is too difficult  you do have the valuable option of using inspect getfullargspec  It offers a similar interface to getargspec  a single callable argument  in order to grab the arguments of a function while also handling some additional cases that getargspec doesn t   from inspect import getfullargspec  def someMethod self  arg1  kwarg1 None       pass  args   getfullargspec someMethod    As with getargspec  getfullargspec returns a NamedTuple which contains the arguments   print args  FullArgSpec args   self    arg1    kwarg1    varargs None  varkw None  defaults  None    kwonlyargs     kwonlydefaults None  annotations

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someMethod func code co argcount   or  if the current function name is undetermined   import sys  sys  getframe   func code co argcount

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func   code   co argcount gives you number of any arguments BEFORE  args   func   kwdefaults   gives you a dict of the keyword arguments AFTER  args   func   code   co kwonlyargcount is equal to len func   kwdefaults      func   defaults   gives you the values of optional arguments that appear before  args   Here is the simple illustration      gt  gt  gt  def a b  c  d  e  f 1  g 3  h None   i  j 2  k 3    L       pass   gt  gt  gt  a   code   co argcount 7  gt  gt  gt  a   defaults    1  3  None   gt  gt  gt  len a   defaults    3  gt  gt  gt    gt  gt  gt    gt  gt  gt  a   kwdefaults     j   2   k   3   gt  gt  gt  len a   kwdefaults    2  gt  gt  gt  a   code   co kwonlyargcount 2

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import inspect inspect getargspec someMethod    see the inspect module

[python] How can I find the number of arguments of a Python function?

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