Python Generators
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What is a Generator?
A generator function looks like an ordinary function but uses yield instead of return, and calling it doesn't run the function body immediately -- it returns a generator object that produces values one at a time only as they're requested.
Example: What is a Generator?
def my_gen():
yield 1
yield 2
g = my_gen()
print(type(g))
print(next(g))The yield Keyword
Each time yield executes, the function pauses exactly there, hands back the yielded value, and remembers its entire local state (variables, position in loops) -- the next call to next() resumes execution right after that yield rather than starting the function over.
Example: The yield Keyword
def counter():
n = 1
while True:
yield n
n += 1
c = counter()
print(next(c))
print(next(c))Generator Expressions
A generator expression -- (x*2 for x in range(10)) -- creates a generator with syntax almost identical to a list comprehension, but using parentheses instead of square brackets, and without ever building the full list of results in memory.
Example: Generator Expressions
squares = (x * x for x in range(5))
print(list(squares))Generator State and Lifecycle
When a generator function runs off the end of its body or hits a plain return statement, Python automatically raises StopIteration to signal there's nothing left to produce -- this is exactly the same signal a for loop watches for when iterating any iterator.
Example: Generator State and Lifecycle
def one_value():
yield "only value"
g = one_value()
print(next(g))
try:
next(g)
except StopIteration:
print("Generator exhausted")Performance Benefits
Because a generator computes and yields one value at a time instead of building an entire list upfront, it uses a fixed, small amount of memory regardless of how many values it ultimately produces -- ideal for streaming through datasets too large to fit comfortably in RAM.
Example: Performance Benefits
import sys
list_version = [x for x in range(1000)]
gen_version = (x for x in range(1000))
print(sys.getsizeof(list_version) > sys.getsizeof(gen_version))Chapter Quiz — Complete all 12 topics to unlock
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