Python Function Arguments: Introduction
The first thing a programmer must be aware of is that parameters and arguments are clearly two different things although people use them synonymously.
Parameters are the variables that are defined or used inside parentheses while defining a function, whereas arguments are the value passed for these parameters while calling a function. Arguments are the values that are passed to the function at run-time so that the function can do the designated task using these values.
Now that you know about Python function arguments and parameters, let’s have a look at a simple program to highlight more before discussing the types of arguments that can be passed to a function.
#Defining a function first to return the value of parameter def display(x): #x is the parameter return x #Calling the function and supplying arguments print ("Hello " + display('David')) #'David' is the argument
In this example, we have defined a function with parameter ‘x’. Basically, this function will return whatever the value is passed as an argument while calling.
Types of function arguments in Python
There are three types of Python function arguments using which we can call a function.
- Default Arguments
- Keyword Arguments
- Variable-length Arguments
Python Default Arguments
Sometimes we may want to use parameters in a function that takes default values in case the user doesn’t want to provide a value for them.
For this, we can use default arguments which assumes a default value if a value is not supplied as an argument while calling the function. In parameters list, we can give default values to one or more parameters.
An assignment operator ‘=’ is used to give a default value to an argument. Here is an example.
def sum(a=4, b=2): #2 is supplied as default argument """ This function will print sum of two numbers if the arguments are not supplied it will add the default value """ print (a+b) sum(1,2) #calling with arguments sum( ) #calling without arguments
In the program above, default arguments 2 and 4 are supplied to the function. First, the user has provided the arguments 1 and 2, hence the function prints their sum which is 3. In the second call, the user has not provided the arguments. Thus the function takes the default arguments and prints their sum.
Why does Python throw error while using default argument before non-default argument?
Well, the reason is simple.
In function, the values are assigned to parameters in order, by their position. The value of default argument may be optional as there is the default value if not provided, but the value for a non-default argument is mandatory as there exists no default value for this.
def func(a=1, b) is not allowed because when we will call the function using
func(5), the default argument
a will be replaced by 5, leaving no value for parameter
b. That is why a non-default argument is always placed before default argument.
Python Keyword Arguments
In function, the values passed through arguments are assigned to parameters in order, by their position.
With Keyword arguments, we can use the name of the parameter irrespective of its position while calling the function to supply the values. All the keyword arguments must match one of the arguments accepted by the function.
Here is an example.
def print_name(name1, name2): """ This function prints the name """ print (name1 + " and " + name2 + " are friends") #calling the function print_name(name2 = 'John',name1 = 'Gary')
Gary and John are friends
Notice in above example, if we had supplied arguments as
print_name('John','Gary'), the output would have been
John and Gary are friends as the values would have been assigned by arguments position.
But using keyword arguments by specifying the name of the argument itself, we don’t have to worry about their position. This makes using function easier as we don’t need to worry about the order of arguments.
Sometimes you may need more arguments to process function then you mentioned in the definition. If we don’t know in advance about the arguments needed in function, we can use variable-length arguments also called arbitrary arguments.
For this an asterisk (*) is placed before a parameter in function definition which can hold non-keyworded variable-length arguments and a double asterisk (**) is placed before a parameter in function which can hold keyworded variable-length arguments.
If we use one asterisk (*) like *var, then all the positional arguments from that point till the end are collected as a tuple
called ‘var’ and if we use two asterisks (**) before a variable like **var, then all the positional arguments from that point till the end are collected as a dictionary called ‘var’.
Here is an example.
def display(*name, **address): for items in name: print (items) for items in address.items(): print (items) #Calling the function display('john','Mary','Nina',John='LA',Mary='NY',Nina='DC')
John Mary Nina ('John', 'LA') ('Mary', 'NY') ('Nina', 'DC')
As you can see in the example above,
*name takes all the non-keyworded arguments
Nina wrapped into a tuple, whereas
**address takes all the keyworded arguments
Mary ='NY', and
Nina='DC' wrapped into a dictionary.
In this particular example, the function took three arbitrary variables using one variable with either asterisk(*) or double asterisk(**), but using variable length arguments we can take any number of arbitrary arguments.