49. Lazy Evaluation with itertools

1. Using itertools.count for Infinite Sequences

Generating an infinite sequence of numbers starting from a specified number using lazy evaluation.

Copy

import itertools

# Infinite sequence of numbers starting from 0
counter = itertools.count(start=0, step=1)

# Get the first 10 numbers from the infinite sequence
for i in itertools.islice(counter, 10):
    print(i)

This generates an infinite sequence of numbers, but we only evaluate the first 10 numbers using islice.

---

2. Using itertools.cycle for Repeated Sequences

Repeating a sequence infinitely with lazy evaluation.

Copy

import itertools

# Infinite cycle of the list ['A', 'B', 'C']
cycle = itertools.cycle(['A', 'B', 'C'])

# Get the first 10 elements from the infinite cycle
for i in itertools.islice(cycle, 10):
    print(i)

cycle generates an infinite loop over a sequence, and islice helps extract a finite number of elements.

---

3. Using itertools.repeat for Repeating an Element

Repeating a single element a specified number of times lazily.

Copy

import itertools

# Repeat 'Hello' 5 times
for word in itertools.repeat('Hello', 5):
    print(word)

repeat will lazily repeat the element the specified number of times, without storing all repeated elements in memory.

---

4. Using itertools.chain to Combine Iterables

Combining multiple iterables lazily without creating a large combined list.

Copy

import itertools

iterable1 = [1, 2, 3]
iterable2 = [4, 5, 6]

# Chain two iterables together lazily
combined = itertools.chain(iterable1, iterable2)

for item in combined:
    print(item)

chain allows us to process the elements of both lists lazily, one by one.

---

5. Using itertools.islice to Slice an Iterable Lazily

Slicing an iterable lazily using islice without consuming memory for the entire sequence.

Copy

import itertools

data = range(100)

# Lazily slice the first 10 elements
sliced_data = itertools.islice(data, 10)

for item in sliced_data:
    print(item)

This returns the first 10 elements from the range(100) lazily, without creating a large list in memory.

---

6. Using itertools.starmap for Applying Functions to Iterable Elements

Using starmap to lazily apply a function to elements of an iterable of tuples.

Copy

import itertools

data = [(2, 3), (4, 5), (6, 7)]

# Lazily apply multiplication function to each tuple
result = itertools.starmap(lambda x, y: x * y, data)

for item in result:
    print(item)

starmap lazily applies the function to each tuple in the iterable.

---

7. Using itertools.combinations for Generating Combinations

Generating combinations of elements lazily, without evaluating the entire set at once.

Copy

import itertools

data = [1, 2, 3, 4]

# Get all 2-combinations of the data lazily
combinations = itertools.combinations(data, 2)

for item in combinations:
    print(item)

This generates combinations of the input data lazily, yielding one combination at a time.

---

8. Using itertools.permutations for Generating Permutations

Generating all permutations of elements lazily.

Copy

import itertools

data = [1, 2, 3]

# Get all permutations of the data lazily
permutations = itertools.permutations(data)

for item in permutations:
    print(item)

permutations generates permutations lazily, and you can use islice if you need only a subset.

---

9. Using itertools.product for Cartesian Product

Generating the Cartesian product of multiple iterables lazily.

Copy

import itertools

iterable1 = [1, 2]
iterable2 = ['A', 'B']

# Get the Cartesian product lazily
product = itertools.product(iterable1, iterable2)

for item in product:
    print(item)

product computes the Cartesian product lazily, generating pairs without creating a large structure.

---

10. Using itertools.combinations_with_replacement for Combinations with Replacement

Generating combinations with replacement lazily.

Copy

import itertools

data = [1, 2, 3]

# Get 2-combinations with replacement lazily
combinations_with_replacement = itertools.combinations_with_replacement(data, 2)

for item in combinations_with_replacement:
    print(item)

This generates combinations with replacement lazily, yielding each combination one by one.

---

These examples illustrate how to leverage itertools for efficient lazy evaluation in Python. By using these iterators, you can handle large datasets or perform computationally expensive operations without consuming too much memory.