Consider the following Python function:
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def func1(val1, val2, val3): return val1 * val2 + val3 # Calling the function. result = func1(3, 4, 6) |
The function, func1 takes three arguments: val1, val2 and val3. In this article, we want to learn how to check if a given argument is passed during the function call.
For example, how do we know if the user passed the val3 argument value?
We will use the following methods:
- Method 1: Using default argument value(s) and if-statement,
- Method 2: Using **kwargs in function definition if-statement
Method 1: Using default argument values and condition statement
The idea is to set a default value for the argument we want to check and then use the if statement to check if the user passed a different value at the function call. Here is an example,
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# The function func1_a defined a default value for val3. def func1_a(val1, val2, val3=7): if val3 != 7: # If the user explicitly passed passes val3, then the default value, # val3=7, is not used. The value passed by the user takes precedence. print("val3 argument was explicitly passed by the user.") else: print("val3 argument was not passed. Default used.") print("val3 is :", val3) # Calling func1_a with three positional arguments, # 3, 4, 6 for val1, val2, val3, respectively. func1_a(3, 4, 6) func1_a(3, 4) |
Output:
val3 argument was explicitly passed by the user. val3 is : 6 val3 argument was not passed. Default used. val3 is : 7
In the example above, val3=7 is used as a default value. If the user passes val3 in the function call, the value passed will take precedence over the default. In the first function call, val3=6 is passed, but in the second call,val3 is not passed, and therefore, the default value is used.
But what if the user explicitly passes val3=7 on the function call? That will be the same as the default; therefore, our test will fail, as shown in the example below.
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func1_a(4, -3, 7) |
Output:
val3 argument was not passed. Default used. val3 is : 7
Important: The idea is to pass a default value that is not expected to be passed by the user. The convention is to use None, as shown in the example below.
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def func1_b(val1, val2, val3=None): if val3 is not None: # if val3 is not None, then it means the argument was passed. print("val3 argument was passed during the function call.") else: # if val3 is None, then it means the default value is being used. # Argument not passed by the user. print("val3 was not passed to the function. Default value is used.") print("val3 is: ", val3) func1_b(3, 4, 6) func1_b(5, 6) |
Output:
val3 argument was passed during the function call. val3 is: 6 val3 was not passed to the function. Default value is used. val3 is: None
In some cases, however, None could be a valid input for your arguments. In such a case, you can use object() as a default value for the argument you want to check. For example,
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# Provide default and check it using if-statement. default_value = object() def func2_b(val1, val2, val3=default_value): if val3 is default_value: print("val3 argument was not passed during the function call.") else: print("val3 was passed to the function") print("val3 is: ", val3) func2_b(2, 4) func2_b(5, 6, 8) |
Output:
val3 argument was not passed during the function call. val3 is: <object object at 0x7f5579e9a4d0> val3 was passed to the function val3 is: 8
Note: Unlike the first two examples, you need to define the object() default outside the function definition. This is because Every time you execute the object() function, you get a unique instance. That is why object() != object(). See below.
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print(None==None) print(object()==object()) |
Output:
True False
That is why the following code yields “Fail”.
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val3 = object() if val3 is object(): print("Pass") else: print("Fail") |
Method 2: Using **kwargs in function definition if-statement
The **kwargs attribute is used to pass a variable number of keyword arguments. When arguments are passed on a function call, the arg-value pairs of those arguments are stored on **kwargs – a dictionary data structure.
Here is how to use **kwargs to check whether an argument is passed.
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def func1(val1, **kwargs): # **kwargs is a convenience dictionary containing argument-value pairs # (argument names are the keys and values are the argument values) # **kwargs will hold all keyword arguments except val1, which is defined explicitly. print(kwargs) if "val3" in kwargs: # If val3 is found in kwargs, it means it was passed during the function call. param = kwargs["val3"] print("val3 argument was passed.") else: print("val3 was not passed.") print("val3 is: ", kwargs.get("val3")) # Passing 4 arguments to func1 func1(val1=3, val2=4, val3=5, val4=6) |
Output:
{'val2': 4, 'val3': 5, 'val4': 6}
val3 argument was passed.
val3 is: 5
Let’s call the same function by passing 3 arguments to it (excluding val3 – the argument we are checking)
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# Passing 3 arguments to func1 func1(val1=3, val2=4, val4=6) |
Output:
{'val2': 4, 'val4': 6}
val3 was not passed.
val3 is: None
Conclusion
This article discussed two methods of checking if an argument in a function is passed. The first method discussed using a default value, and the second explained how to use **kwargs when checking if an argument was passed at a function call.