leejunkim

Generators

4 min read

A specific type of iterator that is created using a generator function (has the yield keyword). It automatically implements the __iter__() and __next__() methods for you. - quick and easy way of creating iterators (no need to manually implement the __iter__() and __next__())

  • Iterator vs Iterable
  • Are a subset of iterators: Every generator is an iterable, but not every iterator is a generator
  • they do not generate all things and store them in the memory but they generate the value on the fly, one at a time
  • can be created with generator functions
    • generator functions use the yield keyword
  • can be created with generator expressions
    • like List Comprehension but using () and not []
    • ex. squares_gen = (x**2 for x in range(10))

Generator Functions

FunctionsGenerator Functions
uses returnuses yield
returns oncecan yield multiple times
when invoked, returns the return valuewhen invoked, returns a generator

Our first generator function that returns a generator

def count_up_to(max):
	count = 1
	while count <= max:
		yield count 
		count += 1
 
count_up_to_(5)
# <generator object count_up_to at 0x102c8ea0>
  • yield
    • pauses the function and saves its state, the function can be resumed later from where it was paused
    • returns the value to the caller
  • When you call count_up_to(), it doesn’t run the function immediately, but it returns a generator. This generator produces values one at a time when you call next() on it or loop over it (e.g., in a for loop).
gen = count_up_to(5)
 
# Call next() to get values from the generator one by one
print(next(gen))  # Output: 1
print(next(gen))  # Output: 2
print(next(gen))  # Output: 3
  • The first next(gen) call runs the function until it hits the yield count, returning 1, then pauses.
  • The second next(gen) call resumes from where it paused, increments count, yields 2, and pauses again.
for num in count_up_to(3):
    print(num)
  • In for loops, Python automatically calls next() on the generator for each iteration of the loop
  • Steps
    • Step 1: At the start of the loop, iter(count_up_to(5)) is called. Since count_up_to() is a generator function, it returns a generator object, which acts as the iterator.
    • Step 2: Python calls next() on that generator to get the first value. The generator yields 1.
    • Step 3: The value 1 is printed.
    • Step 4: The next iteration of the loop calls next() again, which resumes the generator and yields 2, and so on.
    • Step 5: This continues until the generator raises StopIteration after yielding 5, and the loop stops.
counter = count_up_to(10)
list(counter)
# [1,2,3,4,5,6,7,8,9,10]
# just goes through everything and adds to list

Short exercises

"""
Week generator
"""
def week():
    week = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']
    # current = 0
    # while current <= len(week) - 1:
    #     yield week[current]
    #     current += 1
    for day in week:
	    yield week
		# much better than while loop...wtf why didn't I do this?
 
"""
Write a function called **yes_or_no**, which returns a generator that first yields `yes` , then `no` , then `yes` , then `no` , and so on.
"""
 
def yes_or_no():
    while True:
        yield 'yes'
        yield 'no'

Infinite generator

Beat box application lmao

""" Ver 1"""
def current_beat():
    beat = [1,2,3,4]
    cur = 0
    while True:
        if cur == 4: cur = 0
        yield beat[cur]
        cur += 1
 
print(current_beat())
for beat in current_beat():
    print(beat)
 
""" Ver 2 - same thing but shorter """
def current_beat():
    while True:
        yield 1
        yield 2
        yield 3
        yield 4
 
""" Ver 3 - using % (omg....진짜 개천재같음..갈길이 멀다..) """
def current_beat()"
	i = 0
	while True:
		yield i % 4 + 1
		i += 1

Generator Expressions

  • like List Comprehension but using () and not []
    • ex. squares_gen = (x**2 for x in range(10))
def nums():
	for num in range(1,10):
		yield num
 
g = nums()
# <generator object nums at 0x000001>
next(g) # 1
next(g) # 2
 
# Same thing
g = (num for num in range(1,10))
# <generator object <genexpr> at 0x0000100>
next(g) # 1
next(g) # 2
 
# operations still work
sum([num for num in range(1,10)]) # 45 (list comprehension)
sum(num for num in range(1,10)) # 45 (generator expression)

Graph View

Backlinks