Python call user function

Python Functions

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

A function can return data as a result.

Creating a Function

In Python a function is defined using the def keyword:

Example

Calling a Function

To call a function, use the function name followed by parenthesis:

Example

def my_function():
print(«Hello from a function»)

my_function()

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

def my_function(fname):
print(fname + » Refsnes»)

my_function(«Emil»)
my_function(«Tobias»)
my_function(«Linus»)

Arguments are often shortened to args in Python documentations.

Parameters or 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 value that is sent to the function when it is called.

Number of Arguments

By default, a function must be called with the correct number of arguments. Meaning that if your function expects 2 arguments, you have to call the function with 2 arguments, not more, and not less.

Example

This function expects 2 arguments, and gets 2 arguments:

def my_function(fname, lname):
print(fname + » » + lname)

Example

This function expects 2 arguments, but gets only 1:

def my_function(fname, lname):
print(fname + » » + lname)

Arbitrary Arguments, *args

If you do not know how many arguments that will be passed into your function, add a * before the parameter name in the function definition.

This way the function will receive a tuple of arguments, and can access the items accordingly:

Example

If the number of arguments is unknown, add a * before the parameter name:

def my_function(*kids):
print(«The youngest child is » + kids[2])

my_function(«Emil», «Tobias», «Linus»)

Arbitrary Arguments are often shortened to *args in Python documentations.

Keyword Arguments

You can also send arguments with the key = value syntax.

This way the order of the arguments does not matter.

Example

def my_function(child3, child2, child1):
print(«The youngest child is » + child3)

my_function(child1 = «Emil», child2 = «Tobias», child3 = «Linus»)

The phrase Keyword Arguments are often shortened to kwargs in Python documentations.

Arbitrary Keyword Arguments, **kwargs

If you do not know how many keyword arguments that will be passed into your function, add two asterisk: ** before the parameter name in the function definition.

This way the function will receive a dictionary of arguments, and can access the items accordingly:

Example

If the number of keyword arguments is unknown, add a double ** before the parameter name:

def my_function(**kid):
print(«His last name is » + kid[«lname»])

my_function(fname = «Tobias», lname = «Refsnes»)

Arbitrary Kword Arguments are often shortened to **kwargs in Python documentations.

Default Parameter Value

The following example shows how to use a default parameter value.

If we call the function without argument, it uses the default value:

Example

def my_function(country = «Norway»):
print(«I am from » + country)

Passing a List as an Argument

You can send any data types of argument to a function (string, number, list, dictionary etc.), and it will be treated as the same data type inside the function.

E.g. if you send a List as an argument, it will still be a List when it reaches the function:

Example

def my_function(food):
for x in food:
print(x)

fruits = [«apple», «banana», «cherry»]

Return Values

To let a function return a value, use the return statement:

Example

The pass Statement

function definitions cannot be empty, but if you for some reason have a function definition with no content, put in the pass statement to avoid getting an error.

Example

Recursion

Python also accepts function recursion, which means a defined function can call itself.

Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.

The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.

In this example, tri_recursion() is a function that we have defined to call itself («recurse»). We use the k variable as the data, which decrements ( -1 ) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).

To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.

Example

def tri_recursion(k):
if(k > 0):
result = k + tri_recursion(k — 1)
print(result)
else:
result = 0
return result

print(«\n\nRecursion Example Results»)
tri_recursion(6)

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How to Call a Function in Python

This article was co-authored by wikiHow staff writer, Nicole Levine, MFA. Nicole Levine is a Technology Writer and Editor for wikiHow. She has more than 20 years of experience creating technical documentation and leading support teams at major web hosting and software companies. Nicole also holds an MFA in Creative Writing from Portland State University and teaches composition, fiction-writing, and zine-making at various institutions.

This article has been viewed 37,593 times.

This wikiHow teaches you how to define and call a function in a Python script.

Open your Python editor. You can use Idle or any programming editor you have on your computer (including Notes or Notepad).

Choose a name for your function. Choose a descriptive name that explains what your function does. Use lowercase letters and underscores to separate words in the name. Avoid using names that are already used by built-in functions or modules in Python.

Define a function. Use the «def» keyword to define a function in Python. Follow «def» with the name of your function and a set of parentheses. Add a colon at the end of the line. As an example, we’re going to start by defining a function called printme.

Add parameters to the function. If your function needs to take input, add parameters to the parentheses after the function name. Separate multiple parameters with commas. Give each parameter a descriptive name that explains what it represents. Type the following code:

def printme( str ): “This prints a passed string into this function” print str return; 

• Use variables to store values and perform calculations. You can use any valid Python code inside a function.

Add the call to the code. Now that you’ve the defined print me function, you can call it with the code printme(“str”) , where str is whatever it is you want printed out. On the next line after return; , add a printme call as shown (do not indent!):

def printme( str ): “This prints a passed string into this function” print str return; printme(“Hey! How are you doing?”) 

Add a return statement. If your function needs to return a value, use the «return» keyword followed by the value. You can return any value that can be stored in a variable in Python.

Test your function before moving on, test your function to make sure it works as expected. Call the function with different inputs and see if it returns the correct output. You can use print statements to display the output of your function.

Add comments to your code. Use comments to explain what your code does. Start a comment with the «#» symbol. Write comments on a separate line or at the end of a line of code.

Add docstrings to your function. A docstring is a multi-line comment that explains what your function does. Place the docstring directly under the function definition, before the code. Use triple quotes («»» «»» or ) to define a docstring.

Handle errors, Use a «try/except» block to handle errors that could occur in your function. Use «try» to wrap the code that could cause an error. Use «except» to handle the error if it occurs.

Add default values to parameters. You can give parameters default values so that they don’t always need to be specified. Define the default value in the parentheses after the parameter name. Make sure that the parameter with the default value comes after any required parameters.

Use type hints. Type hints specify the expected type of a parameter or return value. Use a colon followed by the type name to add a type hint. Use «->» followed by the type name to add a type hint for the return value.

• Typically you’ll click the File menu, then Save As…, select a folder, type a file name (e.g. printme.py), then click Save.

• Windows: Type cmd into the search bar, then click Command Prompt in the search results.
• macOS: In Finder, open the Applications folder, double-click the Utilities folder, then double-click Terminal.

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