How to get method parameter names

Question

Given the Python function   def a method arg1  arg2       pass   How can I extract the number and names of the arguments  I e   given that I have a reference to func  I want the func  something  to return   arg1    arg2     The usage scenario for this is that I have a decorator  and I wish to use the method arguments in the same order that they appear for the actual function as a key  I e   how would the decorator look that printed  a b  when I call a method  a    b

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Returns a list of argument names  takes care of partials and regular functions   def get func args f       if hasattr f   args            return f args     else          return list inspect signature f  parameters

User · Answer

In CPython  the number of arguments is  a method func code co argcount   and their names are in the beginning of  a method func code co varnames   These are implementation details of CPython  so this probably does not work in other implementations of Python  such as IronPython and Jython   One portable way to admit  pass-through  arguments is to define your function with the signature func  args    kwargs   This is used a lot in e g  matplotlib  where the outer API layer passes lots of keyword arguments to the lower-level API

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In a decorator method  you can list arguments of the original method in this way   import inspect  itertools   def my decorator             def decorator f                def wrapper  args    kwargs                      if you want arguments names as a list                  args name   inspect getargspec f  0                  print args name                     if you want names and values as a dictionary                  args dict   dict itertools izip args name  args                   print args dict                     if you want values as a list                  args values   args dict values                   print args values    If the   kwargs are important for you  then it will be a bit complicated           def wrapper  args    kwargs                args name   list OrderedDict fromkeys inspect getargspec f  0    kwargs keys                 args dict   OrderedDict list itertools izip args name  args     list kwargs iteritems                 args values   args dict values     Example    my decorator   def my function x  y  z 3       pass   my function 1  y 2  z 3  w 0    prints      x    y    z    w       y   2   x   1   z   3   w   0     1  2  3  0

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Take a look at the inspect module - this will do the inspection of the various code object properties for you    gt  gt  gt  inspect getfullargspec a method     arg1    arg2    None  None  None    The other results are the name of the  args and   kwargs variables  and the defaults provided   ie    gt  gt  gt  def foo a  b  c 4   arglist    keywords   pass  gt  gt  gt  inspect getfullargspec foo     a    b    c     arglist    keywords    4      Note that some callables may not be introspectable in certain implementations of Python  For Example  in CPython  some built-in functions defined in C provide no metadata about their arguments  As a result  you will get a ValueError if you use inspect getfullargspec   on a built-in function   Since Python 3 3  you can use inspect signature   to see the call signature of a callable object    gt  gt  gt  inspect signature foo   lt Signature  a  b  c 4   arglist    keywords  gt

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inspect signature is very slow  Fastest way is   def f a  b 1   args  c  d 1    kwargs      pass  f code   f   code   f code co varnames  f code co argcount   f code co kwonlyargcount       a    b    c    d

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To update a little bit Brian s answer  there is now a nice backport of inspect signature that you can use in older python versions  funcsigs  So my personal preference would go for  try     python 3 3      from inspect import signature except ImportError      from funcsigs import signature  def aMethod arg1  arg2       pass  sig   signature aMethod  print sig    For fun  if you re interested in playing with Signature objects and even creating functions with random signatures dynamically you can have a look at my makefun project

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In Python 3   with the Signature object at hand  an easy way to get a mapping between argument names to values  is using the Signature s bind   method   For example  here is a decorator for printing a map like that   import inspect  def decorator f       def wrapper  args    kwargs           bound args   inspect signature f  bind  args    kwargs          bound args apply defaults           print dict bound args arguments            return f  args    kwargs       return wrapper   decorator def foo x  y  param with default  bars     kwargs       pass  foo 1  2  extra  baz     This will print    kwargs     extra    baz     param with default    bars    y   2   x   1

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To update a little bit Brian s answer  there is now a nice backport of inspect signature that you can use in older python versions  funcsigs  So my personal preference would go for  try     python 3 3      from inspect import signature except ImportError      from funcsigs import signature  def aMethod arg1  arg2       pass  sig   signature aMethod  print sig    For fun  if you re interested in playing with Signature objects and even creating functions with random signatures dynamically you can have a look at my makefun project

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Here is something I think will work for what you want  using a decorator   class LogWrappedFunction object       def   init   self  function           self function   function      def logAndCall self   arguments    namedArguments           print  Calling  s with arguments  s and named arguments  s                            self function func name  arguments  namedArguments          self function   call    arguments    namedArguments   def logwrap function       return LogWrappedFunction function  logAndCall   logwrap def doSomething spam  eggs  foo  bar       print  Doing something totally awesome with  s and  s      spam  eggs    doSomething  beans   rice   foo  wiggity   bar  wack     Run it  it will yield the following output   C  scripts gt python decoratorExample py Calling doSomething with arguments   beans    rice   and named arguments   foo     wiggity    bar    wack   Doing something totally awesome with beans and rice

User · Answer

What about dir   and vars   now   Seems doing exactly what is being asked super simply     Must be called from within the function scope   But be wary that it will return all local variables so be sure to do it at the very beginning of the function if needed   Also note that  as pointed out in the comments  this doesn t allow it to be done from outside the scope  So not exactly OP s scenario but still matches the question title  Hence my answer

User · Answer

Take a look at the inspect module - this will do the inspection of the various code object properties for you    gt  gt  gt  inspect getfullargspec a method     arg1    arg2    None  None  None    The other results are the name of the  args and   kwargs variables  and the defaults provided   ie    gt  gt  gt  def foo a  b  c 4   arglist    keywords   pass  gt  gt  gt  inspect getfullargspec foo     a    b    c     arglist    keywords    4      Note that some callables may not be introspectable in certain implementations of Python  For Example  in CPython  some built-in functions defined in C provide no metadata about their arguments  As a result  you will get a ValueError if you use inspect getfullargspec   on a built-in function   Since Python 3 3  you can use inspect signature   to see the call signature of a callable object    gt  gt  gt  inspect signature foo   lt Signature  a  b  c 4   arglist    keywords  gt

User · Answer

Take a look at the inspect module - this will do the inspection of the various code object properties for you    gt  gt  gt  inspect getfullargspec a method     arg1    arg2    None  None  None    The other results are the name of the  args and   kwargs variables  and the defaults provided   ie    gt  gt  gt  def foo a  b  c 4   arglist    keywords   pass  gt  gt  gt  inspect getfullargspec foo     a    b    c     arglist    keywords    4      Note that some callables may not be introspectable in certain implementations of Python  For Example  in CPython  some built-in functions defined in C provide no metadata about their arguments  As a result  you will get a ValueError if you use inspect getfullargspec   on a built-in function   Since Python 3 3  you can use inspect signature   to see the call signature of a callable object    gt  gt  gt  inspect signature foo   lt Signature  a  b  c 4   arglist    keywords  gt

User · Answer

Here is another way to get the function parameters without using any module  def get parameters func       keys   func   code   co varnames  func   code   co argcount    -1      sorter    j  i for i  j in enumerate keys   -1         values   func   defaults     -1      kwargs    i  j for i  j in zip keys  values       sorted args   tuple          sorted  i for i in keys if i not in kwargs   key sorter get            sorted kwargs             i  kwargs i  for i in sorted kwargs keys    key sorter get               return sorted args  sorted kwargs   def f a  b  c  quot hello quot   d  quot world quot    var   a       print get parameters f    Output     a    b      c    hello    d    world

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The Python 3 version is   def  get args dict fn  args  kwargs       args names   fn   code   co varnames  fn   code   co argcount      return    dict zip args names  args      kwargs    The method returns a dictionary containing both args and kwargs

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Here is something I think will work for what you want  using a decorator   class LogWrappedFunction object       def   init   self  function           self function   function      def logAndCall self   arguments    namedArguments           print  Calling  s with arguments  s and named arguments  s                            self function func name  arguments  namedArguments          self function   call    arguments    namedArguments   def logwrap function       return LogWrappedFunction function  logAndCall   logwrap def doSomething spam  eggs  foo  bar       print  Doing something totally awesome with  s and  s      spam  eggs    doSomething  beans   rice   foo  wiggity   bar  wack     Run it  it will yield the following output   C  scripts gt python decoratorExample py Calling doSomething with arguments   beans    rice   and named arguments   foo     wiggity    bar    wack   Doing something totally awesome with beans and rice

User · Answer

Here is another way to get the function parameters without using any module  def get parameters func       keys   func   code   co varnames  func   code   co argcount    -1      sorter    j  i for i  j in enumerate keys   -1         values   func   defaults     -1      kwargs    i  j for i  j in zip keys  values       sorted args   tuple          sorted  i for i in keys if i not in kwargs   key sorter get            sorted kwargs             i  kwargs i  for i in sorted kwargs keys    key sorter get               return sorted args  sorted kwargs   def f a  b  c  quot hello quot   d  quot world quot    var   a       print get parameters f    Output     a    b      c    hello    d    world

User · Answer

Returns a list of argument names  takes care of partials and regular functions   def get func args f       if hasattr f   args            return f args     else          return list inspect signature f  parameters

User · Answer

Python 3 5       DeprecationWarning  inspect getargspec   is deprecated  use inspect signature   instead   So previously   func args   inspect getargspec function  args   Now   func args   list inspect signature function  parameters keys      To test    arg  in list inspect signature function  parameters keys      Given that we have function  function  which takes argument  arg   this will evaluate as True  otherwise as False   Example from the Python console   Python 3 6 0  v3 6 0 41df79263a11  Dec 23 2016  07 18 10   MSC v 1900 32 bit  Intel   on win32  gt  gt  gt  import inspect  gt  gt  gt   iterable  in list inspect signature sum  parameters keys    True

User · Answer

In Python 3   with the Signature object at hand  an easy way to get a mapping between argument names to values  is using the Signature s bind   method   For example  here is a decorator for printing a map like that   import inspect  def decorator f       def wrapper  args    kwargs           bound args   inspect signature f  bind  args    kwargs          bound args apply defaults           print dict bound args arguments            return f  args    kwargs       return wrapper   decorator def foo x  y  param with default  bars     kwargs       pass  foo 1  2  extra  baz     This will print    kwargs     extra    baz     param with default    bars    y   2   x   1

User · Answer

I think what you re looking for is the locals method -    In  6   def test a  b  print locals             In  7   test 1 2                  a   1   b   2

User · Answer

In python 3  below is to make  args and   kwargs into a dict  use OrderedDict for python  lt  3 6 to maintain dict orders    from functools import wraps  def display param func        wraps func      def wrapper  args    kwargs            param   inspect signature func  parameters         all param                 k  args n  if n  lt  len args  else v default             for n   k  v  in enumerate param items    if k     kwargs                    all param  update kwargs          print all param           return func   all param      return wrapper

User · Answer

The Python 3 version is   def  get args dict fn  args  kwargs       args names   fn   code   co varnames  fn   code   co argcount      return    dict zip args names  args      kwargs    The method returns a dictionary containing both args and kwargs

User · Answer

Here is something I think will work for what you want  using a decorator   class LogWrappedFunction object       def   init   self  function           self function   function      def logAndCall self   arguments    namedArguments           print  Calling  s with arguments  s and named arguments  s                            self function func name  arguments  namedArguments          self function   call    arguments    namedArguments   def logwrap function       return LogWrappedFunction function  logAndCall   logwrap def doSomething spam  eggs  foo  bar       print  Doing something totally awesome with  s and  s      spam  eggs    doSomething  beans   rice   foo  wiggity   bar  wack     Run it  it will yield the following output   C  scripts gt python decoratorExample py Calling doSomething with arguments   beans    rice   and named arguments   foo     wiggity    bar    wack   Doing something totally awesome with beans and rice

User · Answer

I think what you re looking for is the locals method -    In  6   def test a  b  print locals             In  7   test 1 2                  a   1   b   2

User · Answer

In CPython  the number of arguments is  a method func code co argcount   and their names are in the beginning of  a method func code co varnames   These are implementation details of CPython  so this probably does not work in other implementations of Python  such as IronPython and Jython   One portable way to admit  pass-through  arguments is to define your function with the signature func  args    kwargs   This is used a lot in e g  matplotlib  where the outer API layer passes lots of keyword arguments to the lower-level API

User · Answer

Update for Brian s answer   If a function in Python 3 has keyword-only arguments  then you need to use inspect getfullargspec   def yay a  b 10     c 20  d 30       pass inspect getfullargspec yay    yields this   FullArgSpec args   a    b    varargs None  varkw None  defaults  10    kwonlyargs   c    d    kwonlydefaults   c   20   d   30   annotations

User · Answer

inspect signature is very slow  Fastest way is   def f a  b 1   args  c  d 1    kwargs      pass  f code   f   code   f code co varnames  f code co argcount   f code co kwonlyargcount       a    b    c    d

User · Answer

Python 3 5       DeprecationWarning  inspect getargspec   is deprecated  use inspect signature   instead   So previously   func args   inspect getargspec function  args   Now   func args   list inspect signature function  parameters keys      To test    arg  in list inspect signature function  parameters keys      Given that we have function  function  which takes argument  arg   this will evaluate as True  otherwise as False   Example from the Python console   Python 3 6 0  v3 6 0 41df79263a11  Dec 23 2016  07 18 10   MSC v 1900 32 bit  Intel   on win32  gt  gt  gt  import inspect  gt  gt  gt   iterable  in list inspect signature sum  parameters keys    True

User · Answer

Here is something I think will work for what you want  using a decorator   class LogWrappedFunction object       def   init   self  function           self function   function      def logAndCall self   arguments    namedArguments           print  Calling  s with arguments  s and named arguments  s                            self function func name  arguments  namedArguments          self function   call    arguments    namedArguments   def logwrap function       return LogWrappedFunction function  logAndCall   logwrap def doSomething spam  eggs  foo  bar       print  Doing something totally awesome with  s and  s      spam  eggs    doSomething  beans   rice   foo  wiggity   bar  wack     Run it  it will yield the following output   C  scripts gt python decoratorExample py Calling doSomething with arguments   beans    rice   and named arguments   foo     wiggity    bar    wack   Doing something totally awesome with beans and rice

User · Answer

Take a look at the inspect module - this will do the inspection of the various code object properties for you    gt  gt  gt  inspect getfullargspec a method     arg1    arg2    None  None  None    The other results are the name of the  args and   kwargs variables  and the defaults provided   ie    gt  gt  gt  def foo a  b  c 4   arglist    keywords   pass  gt  gt  gt  inspect getfullargspec foo     a    b    c     arglist    keywords    4      Note that some callables may not be introspectable in certain implementations of Python  For Example  in CPython  some built-in functions defined in C provide no metadata about their arguments  As a result  you will get a ValueError if you use inspect getfullargspec   on a built-in function   Since Python 3 3  you can use inspect signature   to see the call signature of a callable object    gt  gt  gt  inspect signature foo   lt Signature  a  b  c 4   arglist    keywords  gt

User · Answer

Update for Brian s answer   If a function in Python 3 has keyword-only arguments  then you need to use inspect getfullargspec   def yay a  b 10     c 20  d 30       pass inspect getfullargspec yay    yields this   FullArgSpec args   a    b    varargs None  varkw None  defaults  10    kwonlyargs   c    d    kwonlydefaults   c   20   d   30   annotations

User · Answer

In python 3  below is to make  args and   kwargs into a dict  use OrderedDict for python  lt  3 6 to maintain dict orders    from functools import wraps  def display param func        wraps func      def wrapper  args    kwargs            param   inspect signature func  parameters         all param                 k  args n  if n  lt  len args  else v default             for n   k  v  in enumerate param items    if k     kwargs                    all param  update kwargs          print all param           return func   all param      return wrapper

User · Answer

What about dir   and vars   now   Seems doing exactly what is being asked super simply     Must be called from within the function scope   But be wary that it will return all local variables so be sure to do it at the very beginning of the function if needed   Also note that  as pointed out in the comments  this doesn t allow it to be done from outside the scope  So not exactly OP s scenario but still matches the question title  Hence my answer

User · Answer

In CPython  the number of arguments is  a method func code co argcount   and their names are in the beginning of  a method func code co varnames   These are implementation details of CPython  so this probably does not work in other implementations of Python  such as IronPython and Jython   One portable way to admit  pass-through  arguments is to define your function with the signature func  args    kwargs   This is used a lot in e g  matplotlib  where the outer API layer passes lots of keyword arguments to the lower-level API

User · Answer

In CPython  the number of arguments is  a method func code co argcount   and their names are in the beginning of  a method func code co varnames   These are implementation details of CPython  so this probably does not work in other implementations of Python  such as IronPython and Jython   One portable way to admit  pass-through  arguments is to define your function with the signature func  args    kwargs   This is used a lot in e g  matplotlib  where the outer API layer passes lots of keyword arguments to the lower-level API

User · Answer

In a decorator method  you can list arguments of the original method in this way   import inspect  itertools   def my decorator             def decorator f                def wrapper  args    kwargs                      if you want arguments names as a list                  args name   inspect getargspec f  0                  print args name                     if you want names and values as a dictionary                  args dict   dict itertools izip args name  args                   print args dict                     if you want values as a list                  args values   args dict values                   print args values    If the   kwargs are important for you  then it will be a bit complicated           def wrapper  args    kwargs                args name   list OrderedDict fromkeys inspect getargspec f  0    kwargs keys                 args dict   OrderedDict list itertools izip args name  args     list kwargs iteritems                 args values   args dict values     Example    my decorator   def my function x  y  z 3       pass   my function 1  y 2  z 3  w 0    prints      x    y    z    w       y   2   x   1   z   3   w   0     1  2  3  0

[python] How to get method parameter names?

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