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Python – Get Function Signature

  • Last Updated : 29 Dec, 2020

Let’s consider a scenario where you have written a very lengthy code and want to know the function call details. So what you can do is scroll through your code each and every time for different functions to know their details or you can work smartly. You can create a code where you can get the function details without scrolling through the code. This can be achieved in two ways –

  • Using signature() function
  • Using decorators

Using signature() function

We can get function Signature with the help of signature() Function. It takes callable as a parameter and returns the annotation. It raises a value Error if no signature is provided. If the Invalid type object is given then it throws a Type Error.

Syntax:

inspect.signature(callable, *, follow_wrapped=True)

Example 1:




from inspect import signature
  
  
# declare a function gfg with some
# parameter
def gfg(x:str, y:int):
    pass
  
# with the help of signature function
# store signature of the function in
# variable t
t = signature(gfg)
  
# print the signature of the function
print(t)
  
# print the annonation of the parameter
# of the function
print(t.parameters['x'])
  
# print the annonation of the parameter
# of the function
print(t.parameters['y'].annotation)

Output

(x:str, y:int)
x:str
<class 'int'>

Using Decorators

To do this the functions in Python certain attributes. One such attribute is __code__ that returns the called function bytecode. The __code__ attributes also have certain attributes that will help us in performing our tasks. We will be using the co_varnames attribute that returns the tuple of names of arguments and local variables and co_argcount that returns the number of arguments (not including keyword-only arguments, * or ** args). Let’s see the below implementation of such decorator using these discussed attributes.

Example:




# Decorator to print function call 
# details 
def function_details(func): 
        
        
    # Getting the argument names of the 
    # called function 
    argnames = func.__code__.co_varnames[:func.__code__.co_argcount] 
        
    # Getting the Function name of the 
    # called function 
    fname = func.__name__ 
        
        
    def inner_func(*args, **kwargs): 
            
        print(fname, "(", end = "") 
            
        # printing the function arguments 
        print(', '.join( '% s = % r' % entry 
            for entry in zip(argnames, args[:len(argnames)])), end = ", "
            
        # Printing the variable length Arguments 
        print("args =", list(args[len(argnames):]), end = ", "
            
        # Printing the variable length keyword 
        # arguments 
        print("kwargs =", kwargs, end = "") 
        print(")"
            
    return inner_func 
    
    
# Driver Code 
@function_details
def GFG(a, b = 1, *args, **kwargs): 
    pass
    
GFG(1, 2, 3, 4, 5, d = 6, g = 12.9
GFG(1, 2, 3
GFG(1, 2, d = 'Geeks'

Output:

GFG (a = 1, b = 2, args = [3, 4, 5], kwargs = {‘d’: 6, ‘g’: 12.9})
GFG (a = 1, b = 2, args = [3], kwargs = {})
GFG (a = 1, b = 2, args = [], kwargs = {‘d’: ‘Geeks’})

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