Introduction to Python functions for those new to Python, programming, or both

1. Python Functions
Rick Copeland

2. Python Functions
Rick Copeland

3. Python Functions
Rick Copeland

4. Beginner’s
Night
Python Functions
Rick Copeland

5. What is a function, anyway?

6. What is a function, anyway?
• Reusable bit of Python code

7. What is a function, anyway?
• Reusable bit of Python code
• … beginning with the keyword def

8. What is a function, anyway?
• Reusable bit of Python code
• … beginning with the keyword def
• … that can use parameters (placeholders)

9. 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

10. Define a Function
>>> def x_squared(x):!
... return x * x

11. Use a Function
>>> def x_squared(x):!
... return x * x!
...!
>>> x_squared(1)!
1!
>>> x_squared(2)!
4!
>>> x_squared(4)!
16

12. Use a Function
• To actually execute the
function we’ve defined, we
need to call or invoke it.
>>> def x_squared(x):!
... return x * x!
...!
>>> x_squared(1)!
1!
>>> x_squared(2)!
4!
>>> x_squared(4)!
16

13. 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…
>>> def x_squared(x):!
... return x * x!
...!
>>> x_squared(1)!
1!
>>> x_squared(2)!
4!
>>> x_squared(4)!
16

14. 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
>>> def x_squared(x):!
... return x * x!
...!
>>> x_squared(1)!
1!
>>> x_squared(2)!
4!
>>> x_squared(4)!
16

15. Substitution
x_squared(4)

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

17. Substitution
x_squared(4) >>> def x_squared(x):!
... return x * x
x = 4!
return x * x

18. Substitution
x_squared(4) >>> def x_squared(x):!
... return x * x
x = 4!
return x * x
4 * 4

19. Substitution
x_squared(4) >>> def x_squared(x):!
... return x * x
x = 4!
return x * x
4 * 4
16

20. Recursion

21. Recursion
• Functions can call themselves

22. Recursion
• Functions can call themselves
• … we call this recursion

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

24. Recursion, step-by-step
fact(3)

25. Recursion, step-by-step
fact(3)
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

26. Recursion, step-by-step
fact(3)
return 3 * x_fact(3-1)
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

27. Recursion, step-by-step
fact(3)
(3 * fact(2)) return 3 * x_fact(3-1)
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

28. Recursion, step-by-step
fact(3)
(3 * fact(2)) return 3 * x_fact(3-1)
return 2 * x_fact(2-1)
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

29. Recursion, step-by-step
fact(3)
(3 * fact(2))
(3 * (2 * fact(1)))
return 3 * x_fact(3-1)
return 2 * x_fact(2-1)
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

30. Recursion, step-by-step
fact(3)
(3 * fact(2))
(3 * (2 * fact(1)))
return 3 * x_fact(3-1)
return 2 * x_fact(2-1)
return 1
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

31. Recursion, step-by-step
fact(3)
(3 * fact(2))
(3 * (2 * fact(1)))
(3 * (2 * (1)))
return 3 * x_fact(3-1)
return 2 * x_fact(2-1)
return 1
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

32. Recursion, step-by-step
fact(3)
(3 * fact(2))
(3 * (2 * fact(1)))
(3 * (2 * (1)))
(3 * (2))
return 3 * x_fact(3-1)
return 2 * x_fact(2-1)
return 1
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

33. Recursion, step-by-step
fact(3)
(3 * fact(2))
(3 * (2 * fact(1)))
(3 * (2 * (1)))
(3 * (2))
return 3 * x_fact(3-1)
return 2 * x_fact(2-1)
return 1
(6)
def x_fact(x):!
if x < 2:!
return 1!
else:!
return x * x_fact(x-1)

34. Different ways to call
functions

35. Different ways to call
functions
def my_awesome_function(something): …

36. Different ways to call
functions
def my_awesome_function(something): …

37. Different ways to call
functions
def my_awesome_function(something): …
def my_awesome_function(something, something_else): …

38. Different ways to call
functions
def my_awesome_function(something): …
def my_awesome_function(something, something_else): …

39. 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): …

40. 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): …

41. 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): …

42. 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): …

43. 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): ...

44. 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?!

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

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

47. 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

48. Default Arguments

49. Default Arguments
def my_awesome_function(something, something_else,

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

51. Default Arguments
def my_awesome_function(something, something_else,
banana=1, apple=2, pear=3): ...
Now when you say this:

52. Default Arguments
def my_awesome_function(something, something_else,
banana=1, apple=2, pear=3): ...
Now when you say this:
>>> my_awesome_function(1, 4, 6)

53. Default Arguments
def 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:

54. Default Arguments
def 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)

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

56. 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)…

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

58. Variable Parameters
• Python “packs” all the
remaining arguments into a
tuple
def sum(*values):!
result = 0!
for v in values:!
result += v!
return result

59. Variable Parameters
• Python “packs” all the
remaining arguments into a
tuple
• You can then pass any
number of positional
arguments to the function
def sum(*values):!
result = 0!
for v in values:!
result += v!
return result

60. Variable Parameters
• Python “packs” all the
remaining arguments into a
tuple
• You can then pass any
number of positional
arguments to the function
def sum(*values):!
result = 0!
for v in values:!
result += v!
return result
A tuple is like a list you
can’t modify, e.g. (1,2,3)

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

62. 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

63. Variable Arguments
• Python “unpacks” the tuple/
list/etc. into separate
arguments
• You can call functions that use
variable or fixed arguments
this way
>>> def sum(*values):!
... result = 0!
... for v in values:!
... result += v!
... return result!
...!
>>> sum(*[1,2,3])!
6

64. Variable Arguments
• Python “unpacks” the tuple/
list/etc. into separate
arguments
• You can call functions that use
variable or fixed arguments
this way
>>> def sum(*values):!
... result = 0!
... for v in values:!
... result += v!
... return result!
...!
>>> sum(*[1,2,3])!
6
>>> def x_times_y(x, y):!
... return x * y!
...!
>>> x_times_y(*[4,2])!
8

65. Keyword Parameters

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

67. 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…

68. Keyword Parameters

69. 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}

70. Keyword Parameters
• Python “packs” all the
remaining named arguments
into a dict
>>> 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}

71. Keyword Parameters
• Python “packs” all the
remaining named arguments
into a dict
• You can then pass any
number of named arguments
to the function
>>> 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}

72. Keyword Parameters
• Python “packs” all the
remaining named arguments
into a dict
• 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})
>>> 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}

73. Keyword Parameters Step by
Step
>>> make_dict(a=5, b=6)

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

75. Keyword Parameters Step by
Step
>>> 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)

76. Keyword Parameters Step by
Step
>>> 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

77. Keyword Parameters Step by
Step
>>> 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

78. Keyword Parameters Step by
Step
>>> 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}

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

80. Keyword Arguments
• Python “unpacks” the dict into
separate arguments
>>> make_dict(**{'a': 10, 'b': 20})!
{'a': 10, 'b': 20}

81. Keyword Arguments
• Python “unpacks” the dict into
separate arguments
• You can call functions that use
keyword or fixed arguments
this way
>>> make_dict(**{'a': 10, 'b': 20})!
{'a': 10, 'b': 20}

82. Keyword Arguments
>>> make_dict(**{'a': 10, 'b': 20})!
{'a': 10, 'b': 20}
• Python “unpacks” the dict into
separate arguments
• You can call functions that use
keyword or fixed arguments
this way
>>> dct = {'x': 2, 'y': 6}!
>>> x_times_y(**dct)!
12

83. Keyword Arguments Step by
Step
>>> dct = {'x': 2, 'y': 6}!
>>> x_times_y(**dct)

84. Keyword Arguments Step by
Step
>>> dct = {'x': 2, 'y': 6}!
>>> x_times_y(**dct)
def x_times_y(x, y):!
return x * y

85. Keyword Arguments Step by
Step
>>> dct = {'x': 2, 'y': 6}!
>>> x_times_y(**dct)
x_times_y(x=2, y=6)
def x_times_y(x, y):!
return x * y

86. Keyword Arguments Step by
Step
>>> dct = {'x': 2, 'y': 6}!
>>> x_times_y(**dct)
x_times_y(x=2, y=6)
def x_times_y(x, y):!
return x * y
x=2!
y=6!
return x * y

87. Keyword Arguments Step by
Step
>>> dct = {'x': 2, 'y': 6}!
>>> x_times_y(**dct)
x_times_y(x=2, y=6)
def x_times_y(x, y):!
return x * y
x=2!
y=6!
return x * y
return 2 * 6

88. Keyword Arguments Step by
Step
>>> dct = {'x': 2, 'y': 6}!
>>> x_times_y(**dct)
x_times_y(x=2, y=6)
def x_times_y(x, y):!
return x * y
x=2!
y=6!
return x * y
return 2 * 6
12

89. Is there more?

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

91. Is there more?
• Of course there’s more, but it’s beginner’s night ;-)
• But what can I do now?

92. 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)

93. 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

94. 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)

97. “Any questions?”
–Rick Copeland