Python Function Arguments
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Arguments
Information can be passed into functions as arguments.
Arguments are specified after the function name, inside the parentheses.
You can add as many arguments as you want, just separate them with a comma.
The following example has a function with one argument (fname).
When the function is called, we pass along a first name,
which is used inside the function to print the full name:
Example
A function with one argument:
def my_function(fname):
print(fname + " Refsnes")
my_function("Emil")
my_function("Tobias")
my_function("Linus")
Try it Yourself »
Parameters vs Arguments
The terms parameter and argument can be used for the same thing: information that are passed into a function.
From a function's perspective:
A parameter is the variable listed inside the parentheses in the function definition.
An argument is the actual value that is sent to the function when it is called.
Example
def my_function(name): # name is a parameter
print("Hello", name)
my_function("Emil") # "Emil" is an argument
Number of Arguments
By default, a function must be called with the correct number of arguments.
If your function expects 2 arguments, you must call it with exactly 2 arguments.
Example
This function expects 2 arguments, and gets 2 arguments::
def my_function(fname, lname):
print(fname + " " + lname)
my_function("Emil", "Refsnes")
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If you try to call the function with the wrong number of arguments, you will get an error:
Example
This function expects 2 arguments, but gets only 1:
def my_function(fname, lname):
print(fname + " " + lname)
my_function("Emil")
Try it Yourself »
Default Parameter Values
You can assign default values to parameters. If the function is called without an argument, it uses the default value:
Example
def my_function(name = "friend"):
print("Hello", name)
my_function("Emil")
my_function("Tobias")
my_function()
my_function("Linus")
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Example
Default value for country parameter:
def my_function(country = "Norway"):
print("I am from", country)
my_function("Sweden")
my_function("India")
my_function()
my_function("Brazil")
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Keyword Arguments
You can send arguments with the key = value syntax.
Example
def my_function(animal, name):
print("I have a", animal)
print("My", animal + "'s name is", name)
my_function(animal = "dog", name = "Buddy")
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This way, with keyword arguments, the order of the arguments does not matter.
Example
def my_function(animal, name):
print("I have a", animal)
print("My", animal + "'s name is", name)
my_function(name = "Buddy", animal = "dog")
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The phrase Keyword Arguments is often shortened to kwargs in Python documentation.
Positional Arguments
When you call a function with arguments without using keywords, they are called positional arguments.
Positional arguments must be in the correct order:
Example
def my_function(animal, name):
print("I have a", animal)
print("My", animal + "'s name is", name)
my_function("dog", "Buddy")
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The order matters with positional arguments:
Example
Switching the order changes the result:
def my_function(animal, name):
print("I have a", animal)
print("My", animal + "'s name is", name)
my_function("Buddy", "dog")
Try it Yourself »
Mixing Positional and Keyword Arguments
You can mix positional and keyword arguments in a function call.
However, positional arguments must come before keyword arguments:
Example
def my_function(animal, name, age):
print("I have a", age, "year old", animal, "named", name)
my_function("dog", name = "Buddy", age = 5)
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Passing Different Data Types
You can send any data type as an argument to a function (string, number, list, dictionary, etc.).
The data type will be preserved inside the function:
Example
Sending a list as an argument:
def my_function(fruits):
for fruit in fruits:
print(fruit)
my_fruits = ["apple", "banana", "cherry"]
my_function(my_fruits)
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Example
Sending a dictionary as an argument:
def my_function(person):
print("Name:", person["name"])
print("Age:", person["age"])
my_person = {"name": "Emil", "age": 25}
my_function(my_person)
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Return Values
Functions can return values using the return statement:
Example
def my_function(x, y):
return x + y
result = my_function(5, 3)
print(result)
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Returning Different Data Types
Functions can return any data type, including lists, tuples, dictionaries, and more.
Example
A function that returns a list:
def my_function():
return ["apple", "banana", "cherry"]
fruits = my_function()
print(fruits[0])
print(fruits[1])
print(fruits[2])
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Example
A function that returns a tuple:
def my_function():
return (10, 20)
x, y = my_function()
print("x:", x)
print("y:", y)
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Positional-Only Arguments
You can specify that a function can have ONLY positional arguments.
To specify positional-only arguments, add , / after the arguments:
Example
def my_function(name, /):
print("Hello", name)
my_function("Emil")
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Without the , / you are actually allowed to use keyword arguments even if the function
expects positional arguments:
Example
def my_function(name):
print("Hello", name)
my_function(name = "Emil")
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With , /, you will get an error if you try to use keyword arguments:
Example
def my_function(name, /):
print("Hello", name)
my_function(name = "Emil")
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Keyword-Only Arguments
To specify that a function can have only keyword arguments, add *, before the arguments:
Example
def my_function(*, name):
print("Hello", name)
my_function(name = "Emil")
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Without *,, you are allowed to use positional arguments even if the function expects keyword arguments:
Example
def my_function(name):
print("Hello", name)
my_function("Emil")
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With *,, you will get an error if you try to use positional arguments:
Example
def my_function(*, name):
print("Hello", name)
my_function("Emil")
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Combining Positional-Only and Keyword-Only
You can combine both argument types in the same function.
Arguments before / are positional-only, and arguments after * are keyword-only:
Example
def my_function(a, b, /, *, c, d):
return a + b + c + d
result = my_function(5, 10, c = 15, d = 20)
print(result)
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