Python Iterators & Generators

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In the Python world, Iterators and Generators are keys to memory efficiency. They allow you to process large amounts of data (even infinite) without having to load everything into RAM at once.

Imagine you want to read a thick book. An iterator allows you to read page by page (loading only one page into memory), instead of trying to memorize a 1000-page book at once.

1. Iterators

An iterator is an object that contains a countable number of values. An iterator can be iterated upon, meaning you can traverse through all the values.

Technically, in Python, an iterator is an object which implements the iterator protocol, which consists of the methods __iter__() and __next__().

Example Creating an Iterator

Let's create a simple iterator that returns numbers starting from 1 up to a certain limit.

class MyNumbers:
    def __init__(self, limit):
        self.limit = limit
        self.num = 1

    def __iter__(self):
        return self

    def __next__(self):
        if self.num <= self.limit:
            x = self.num
            self.num += 1
            return x
        else:
            raise StopIteration

myclass = MyNumbers(3)
myiter = iter(myclass)

print(next(myiter)) # Output: 1
print(next(myiter)) # Output: 2
print(next(myiter)) # Output: 3
# print(next(myiter)) # Will raise StopIteration error

When you use a for loop, Python automatically handles __iter__() and the StopIteration exception.

for x in MyNumbers(3):
    print(x)

2. Generators

Generators are a simple way of creating iterators. Instead of writing a long class with __iter__() and __next__(), you simply define a regular function and use the yield keyword where you want to return data.

Each time yield is called, the function "pauses", saving all its variables, and resumes from that point when called again.

Simple Generator Example

def number_generator(limit):
    num = 1
    while num <= limit:
        yield num
        num += 1

gen = number_generator(3)
# Generator is also an iterator!
print(next(gen)) # 1
print(next(gen)) # 2
print(next(gen)) # 3

Generator Advantage: Memory Efficiency

Imagine you need to process 1 million numbers.

Using List (Consumes Memory):

def get_list():
    result = []
    for i in range(1000000):
        result.append(i)
    return result

# This will consume memory around 40MB+ for list of integers

Using Generator (Saves Memory):

def get_generator():
    for i in range(1000000):
        yield i

# This consumes almost no extra memory, because numbers are generated one by one when requested.

3. Generator Expression

Similar to List Comprehension, but using regular parentheses (). It returns a generator object, not a list.

# List Comprehension (Creates full list in memory)
squares_list = [x**2 for x in range(10)]
print(squares_list) # [0, 1, 4, ..., 81]

# Generator Expression (Lazy evaluation)
squares_gen = (x**2 for x in range(10))
print(squares_gen) # <generator object ...>

# To see content, must iterate
for i in squares_gen:
    print(i, end=" ")

4. Case Study: Reading Large Files

If you have to process a 10GB server log file.

Wrong (Don't do this):

def read_file_wrong(filename):
    file = open(filename)
    content = file.read() # DANGER! Will load entire 10GB to RAM.
    return content.split("\n")

Right (Use Generator):

def read_file_right(filename):
    for line in open(filename):
        yield line

# We can loop through 10GB file without memory issues
for line in read_file_right("server.log"):
    if "ERROR" in line:
        print(line)

Conclusion

Iterator: An object that can be iterated (__next__).
Generator: A function that produces (yield) values one by one (lazy evaluation).
Use Generators when working with large datasets or infinite data streams.

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