Python Functions (PyAtl Beginners Night)

Python FunctionsRick Copeland

Beginner’s

What is a function, anyway?

• Reusable bit of Python code

• … beginning with the keyword def

• … that can use parameters (placeholders)

• … that can provide a return value

Define a Function

>>> def x_squared(x):!... return x * x

Use a Function

>>> def x_squared(x):!... return x * x!...!>>> x_squared(1)!1!>>> x_squared(2)!4!>>> x_squared(4)!16

Use a Function• To actually execute the

function we’ve defined, we need to call or invoke it.

• In Python, we call functions by placing parentheses after the function name…

• … providing arguments which match up with the parameters used when defining the function

Substitutionx_squared(4)

x = 4!return x * x

Recursion

• Functions can call themselves

Recursion

• Functions can call themselves

• … we call this recursion

Recursion>>> def x_fact(x):!... if x < 2:!... return 1!... else:!... return x * x_fact(x-1)!...!>>> x_fact(3)!6

Recursion, step-by-stepfact(3)

def x_fact(x):! if x < 2:! return 1! else:! return x * x_fact(x-1)

return 3 * x_fact(3-1)

(3 * fact(2)) return 3 * x_fact(3-1)

(3 * fact(2))

(3 * (2 * fact(1)))

(3 * fact(2))

(3 * (2 * fact(1)))

return 1

(3 * fact(2))

(3 * (2 * fact(1)))

(3 * (2 * (1)))

return 1

(3 * fact(2))

(3 * (2 * fact(1)))

(3 * (2 * (1)))

(3 * (2))

return 1

(3 * fact(2))

(3 * (2 * fact(1)))

(3 * (2 * (1)))

(3 * (2))

return 1

Different ways to call functions

def my_awesome_function(something): …

def my_awesome_function(something, something_else): …

def my_awesome_function(something, something_else, banana): …

def my_awesome_function(something, something_else, banana, apple): …

def my_awesome_function(something, something_else, banana, apple, pear): ...

>>> my_awesome_function(1, 4, 6, ... # now was it banana, apple, pear or apple, pear, banana?!

Named Arguments>>> my_awesome_function(! something=1,! something_else=4,! banana=6,! pear=9,! apple=12)

Default Arguments

• When you’re calling a function, you have to give Python all the argument values (1 per parameter)

Default Arguments

• When you’re calling a function, you have to give Python all the argument values (1 per parameter)

• … but some of these can have default values

Default Arguments

Default Argumentsdef my_awesome_function(something, something_else,

Default Argumentsdef my_awesome_function(something, something_else, banana=1, apple=2, pear=3): ...

Now when you say this:

>>> my_awesome_function(1, 4, 6)

It means this:

>>> my_awesome_function(! 1, 4, 6,! apple=2, pear=3)

Variable Parameters

• Sometimes it’s nice to be able to call a function with different numbers of arguments…

Variable Parameters

• Sometimes it’s nice to be able to call a function with different numbers of arguments…

• … something like sum(1,2,3) or sum(1,2)…

Variable Parameters

def sum(*values):! result = 0! for v in values:! result += v! return result

Variable Parameters

• Python “packs” all the remaining arguments into a tuple

Variable Parameters

• You can then pass any number of positional arguments to the function

Variable Parameters

• You can then pass any number of positional arguments to the function

A tuple is like a list you can’t modify, e.g. (1,2,3)

Variable Arguments>>> def sum(*values):!... result = 0!... for v in values:!... result += v!... return result!...!>>> sum(*[1,2,3])!6

Variable Arguments

• Python “unpacks” the tuple/list/etc. into separate arguments

>>> def sum(*values):!... result = 0!... for v in values:!... result += v!... return result!...!>>> sum(*[1,2,3])!6

Variable Arguments

• You can call functions that use variable or fixed arguments this way

Variable Arguments

• You can call functions that use variable or fixed arguments this way

>>> def x_times_y(x, y):!... return x * y!...!>>> x_times_y(*[4,2])!8

Keyword Parameters

• Sometimes I want a function, but I don’t know what I want to name the arguments…

Keyword Parameters

• Sometimes I want a function, but I don’t know what I want to name the arguments…

• … hard to come up with a really simple example, but hopefully this makes sense…

Keyword Parameters

>>> def make_dict(**kwargs):!... result = {}!... for k, v in kwargs.items():!... result[k] = v!... return result!...!>>> make_dict(a=5, b=6)!{'a': 5, 'b': 6}

Keyword Parameters

• Python “packs” all the remaining named arguments into a dict

Keyword Parameters

• You can then pass any number of named arguments to the function

Keyword Parameters

• You can then pass any number of named arguments to the function

A dict is like a directory mapping “keys” to

“values” (e.g. {key: value})

Keyword Parameters Step by Step

>>> make_dict(a=5, b=6)

>>> make_dict(a=5, b=6) def make_dict(**kwargs):! result = {}! for k, v in kwargs.items():! result[k] = v! return result

kwargs = {'a': 5, 'b': 6}!make_dict(**kwargs)

kwargs = {'a': 5, 'b': 6}!result = {}!for k, v in {'a': 5, 'b': 6}.items():! result[k] = v!return result

result = {}!result = {'a': 5}!result = {'a': 5, 'b': 6}!return result

{'a': 5, 'b': 6}

Keyword Arguments>>> make_dict(**{'a': 10, 'b': 20})!{'a': 10, 'b': 20}

Keyword Arguments

• Python “unpacks” the dict into separate arguments

>>> make_dict(**{'a': 10, 'b': 20})!{'a': 10, 'b': 20}

Keyword Arguments

• You can call functions that use keyword or fixed arguments this way

>>> make_dict(**{'a': 10, 'b': 20})!{'a': 10, 'b': 20}

Keyword Arguments

• You can call functions that use keyword or fixed arguments this way

>>> dct = {'x': 2, 'y': 6}!>>> x_times_y(**dct)!12

>>> make_dict(**{'a': 10, 'b': 20})!{'a': 10, 'b': 20}

Keyword Arguments Step by Step

>>> dct = {'x': 2, 'y': 6}!>>> x_times_y(**dct)

def x_times_y(x, y):! return x * y

x_times_y(x=2, y=6)

x=2!y=6!return x * y

x_times_y(x=2, y=6)

return 2 * 6

x_times_y(x=2, y=6)

return 2 * 6

Is there more?

Is there more?• Of course there’s more, but it’s beginner’s night ;-)

• But what can I do now?

• Write functions to reuse code (DRY)

• Write recursive functions

• Create and use functions with different kinds of arguments and parameters (named, *args, **kwargs)

–Rick Copeland

“Any questions?”

Python Functions (PyAtl Beginners Night)

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