Generators in Python

References: Python Wiki, WikiDocs, [Book] Effective Python

What is Generator in Python?

A construct used to create iterators directly

If a function contains the yield keyword, that function becomes a Generator

A type of iterator returned when a generator function is called
A generator function looks similar to a regular function, but uses yield statements to return data at a desired point and resume processing
While a regular function has a single entry point, a generator can be thought of as having multiple entry points
- Using generators allows you to receive desired data at desired points in time
An Iterator requires implementing __iter__, __next__, or __getitem__ methods in a class, but
- A Generator only requires using the yield keyword inside a function
Generators can be written much more simply than Iterators
Generators are sometimes called "producers"

Calling a Generator function returns a Generator object

ex)

def generator_example():
    print('Print 1')
    print('Print 2')
    yield

g = generator_example()
print(g)
next(g)

Result

When yield is used inside a function, the function becomes a Generator, and a value (variable) is specified with yield

ex)

def number_generator():
    yield 0
    yield 1
    yield 2

for number in number_generator():
    print(number)

Result

Iterators can be simply implemented using just yield in a function
While an Iterator directly returns values via return in the __next__ method,
- In a Generator, the value specified with yield becomes the return value of the __next__ method
While an Iterator explicitly raises a StopIteration exception using raise,
- A Generator automatically raises a StopIteration exception when the end of the function is reached
Each time the built-in __next__ method is called on a generator object, the yield code inside the function is executed, producing a value at yield
- That's why it's called a Generator!
The value passed to yield in a generator is returned to the calling side of the iterator
- The iterator returned from a Generator is a set of values passed to the yield expressions inside the generator function
A Generator can pass values outside the function using yield without ending the function
- return ends the function immediately upon returning,
- yield temporarily yields control so that code outside the function can execute and take the value, then resumes executing the code inside the Generator

When a generator function containing a yield statement is executed, a generator object is returned, but the function's content is not executed at this point.
The generator can be executed through the built-in method next(), which internally takes an iterator as an argument and calls the iterator's __next__() method.
When __next__() is called for the first time, the function's content is executed until it encounters a yield statement, at which point processing is suspended.
At this time, all local state is preserved, including variable states, instruction pointers, internal stack, and exception handling state.
Control is then yielded to the upper context, and when __next__() is called again, the generator resumes from the point where it was suspended.

Using generators is clearer to understand than returning a list of accumulated results
Since generators don't store all inputs and outputs in memory, they can produce continuous output even when the amount of input is hard to determine
Using generators, only the values needed at that moment are received through yield, so there is no need to keep all values in memory

However, since generators produce values on demand and don't remember them, while list a can be used multiple times, generator b is exhausted after a single use
This is true for all iterators - Lists and Sets are iterable but are not iterators, so they are not exhausted