Python - advanced Python

Argument Lists

Variable Argument Lists

def custom_print(*args):
    for i in args:
        print("output:", i)

Keyword Arguments

def custom_print2(**kwargs):
    for k,v in kwargs.items():
        print("{}:{}".format(k,v))


>>>custom_print2(ocelot="awesome", warbler="bird")

ocelot:awesome
warbler:bird

Star Magic

func1(a=10),c=12,b=13)
args = ["apple", "banana", "dog"]

func1(args) #not enough parameters

func1(*args) #a is "apple", b is "banana", c is "dog"

• Length of list must agree with number of parameters to function

d = {'a':10,'b':12, 'c':13}

func1(d) #not enough parameters

func1(**d) #a to 10, b to 12, c to 13
           #based on keyword, NOT position

Tips and Tricks

• Don’t use star magic without a good reason

• Star magic will not be required for CPL assignments

• Keyword parameters must come after positional parameters

• Arbitrary arg list comes after positional args, but before kwargs

• Arbitrary keyword arg lists come after listed keyword args

  • Ex: def func(a, b, *args, d=10, e=11, **kwargs)

Generator Functions

• Easy way to make a custom iterable

• When called, returns an object of type ‘generator’

• Like any iteragle, yields one item at a time

• Determines next item on-the-fly

• Cannot index or slice generator objects

def count_forever(start=0):
    i = start
    while True:
        yield i
        i += 1

for x in count_forever():  #prints numbers up to ten
    print(x)
    if x == 10:
        break

• If execution of generator stops, acts just like end of list
  • In context of loop

def to_binary(it): #would have solved coding interview challenge in 3 lines...
    for x in it:
        yield bin(x)

g = count_to(10)
for x in g:
    print(x)

for y in g:
    print(x) #nothing will print, generator depleted

• Once generators are depleted, they can no longer return
  • This can be avoided by calling the function, rather than assiging and referring to an object

def first_n(num,it):
    '''Yields the first num items from it.

    '''
    counter = 0
    for x in it:
        if counter < num:
            yield x
            counter += 1
        else:
            break

Generator Expressions

nums = (bin(x) for x in it) #it is an iterable, nums is a generator

nums = (bin(x) for x in range(5))

• Generator expressions can have filters:

nums = (bin(x) for x in range(5) if x % 2 == 0)

• Generator expressions can have multiple returns:

g = ((x,y) for x in range(3) for y in range(3))

This will act like nested for loops

g = ((x,y) for x in range(3) for y in range(x))

This makes weird garbage

• You can generate generators:

g = (((x,y) for x in range(3)) for y in range (3))

List Comprehensions

[int(x) for x in ["10", "20"]]

List comprehensions are like generators that return lists

Dict Comprehensions

{k: k.upper() for k in ["a", "b"]}

This is a thing I didn’t know I needed.