CODEWITHSUNDEEP
pythongeneratoryield
Peter Tsung
Peter Tsung

Reputation: 945

where does the yield store value in python

I understand generator generates value once at a time, which could save a lot memory and not like list which stores all value in memory.

I want to know in python, how yield knows which value should be returned during the iteration without storing all data at once in memory?

In my understanding, if i want to print 1 to 100 using yield, it is necessary that yield needs to know or store 1 to 100 first and then move point one by one to return value ?

If not, then how yield return value once at a time, but without storing all value in memory?

Upvotes: 3

Views: 2687

Answers (2)

ShadowRanger
ShadowRanger

Reputation: 155724

Each time a generator yields, the stack frame of the generator is saved off inside the generator object, so it can be restored when execution resumes (when the next value is requested).

You can see the structure definition on CPython here.

If you want to see more, generators are somewhat introspectable, so you can take a look at, say, the progression of the locals, the line number it's currently on, etc.:

def myrange(n):
    for i in range(n):
        yield i

mr = myrange(10)

# Before any values consumed:
print(mr.gi_frame.f_locals)  # Outputs {'n': 10}
print(mr.gi_frame.f_lineno)  # Outputs 1

next(mr)  # Advance one

print(mr.gi_frame.f_locals)  # Outputs {'n': 10, 'i': 0}
print(mr.gi_frame.f_lineno)  # Outputs 3

list(mr)  # Consumes generator

print(mr.gi_frame)  # Outputs None; the frame is discarded when the generator finishes

Upvotes: 2

Ajax1234
Ajax1234

Reputation: 71471

Simply put, yield delays the execution but remembers where it left off. However, more specifically, when yield is called, the variables in the state of the generator function are saved in a "frozen" state. When yield is called again, the built in next function sends back the data in line to be transmitted. If there is no more data to be yielded (hence a StopIteration is raised), the generator data stored in its "frozen" state is discarded.

Upvotes: 3

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