Advanced Argument Passing

This is a very, very nifty Python feature – it really lets you write dynamic programs.

Keyword arguments

When defining a function, you can specify only what you need – in any order

In [151]: def fun(x=0, y=0, z=0):
        print(x,y,z)
   .....:
In [152]: fun(1,2,3)
1 2 3
In [153]: fun(1, z=3)
1 0 3
In [154]: fun(z=3, y=2)
0 2 3

A Common Idiom:

def fun(x, y=None):
    if y is None:
        do_something_different
    go_on_here

Can set defaults to variables

In [156]: y = 4
In [157]: def fun(x=y):
    print("x is:", x)
   .....:
In [158]: fun()
x is: 4

Defaults are evaluated when the function is defined

In [156]: y = 4
In [157]: def fun(x=y):
    print("x is:", x)
   .....:
In [158]: fun()
x is: 4
In [159]: y = 6
In [160]: fun()
x is: 4

This is a very important point.

Function arguments in variables

When a function is called, its arguments are really just:

• a tuple (positional arguments)
• a dict (keyword arguments)

def f(x, y, w=0, h=0):
    print("position: {}, {} -- shape: {}, {}".format(x, y, w, h))

position = (3,4)
size = {'h': 10, 'w': 20}

>>> f(*position, **size)
position: 3, 4 -- shape: 20, 10

Function parameters in variables

You can also pull the parameters out in the function as a tuple and a dict:

def f(*args, **kwargs):
    print("the positional arguments are:", args)
    print("the keyword arguments are:", kwargs)

In [389]: f(2, 3, this=5, that=7)
the positional arguments are: (2, 3)
the keyword arguments are: {'this': 5, 'that': 7}

This can be very powerful...

Passing a dict to str.format()

Now that you know that keyword args are really a dict, you know how this nifty trick works:

The string format() method takes keyword arguments:

In [24]: "My name is {first} {last}".format(last="Barker", first="Chris")
Out[24]: 'My name is Chris Barker'

Build a dict of the keys and values:

In [25]: d = {"last":"Barker", "first":"Chris"}

And pass to format()``with ``**

In [26]: "My name is {first} {last}".format(**d)
Out[26]: 'My name is Chris Barker'

Kinda handy for the dict lab, eh?

This:

print("{} is from {}, and he likes "
      "{} cake, {} fruit, {} salad, "
      "and {} pasta.".format(food_prefs["name"],
                             food_prefs["city"],
                             food_prefs["cake"],
                             food_prefs["fruit"],
                             food_prefs["salad"],
                             food_prefs["pasta"]))

Becomes:

print("{name} is from {city}, and he likes "
      "{cake} cake, {fruit} fruit, {salad} salad, "
      "and {pasta} pasta.".format(**food_prefs))

LAB

Time to play with all this to get a feel for it.

Args and Kwargs

This is not all that clearly specified. The goal is for you to experiment with various ways to define and call functions so that you understand what is possible and what happens with each call.