callable() / iter() / next()

# Checks whether an object is callable (a function, class, etc.).
callable(object)

# Creates an iterator from an iterable.
iter(object)
iter(callable_object, sentinel)

# Returns the next element from an iterator.
next(iterator, default)

# Returns the hash value of an object.
hash(object)

# Use callable() to check whether an object is callable.
def greet():
    return "Hello"

print(callable(greet))    # True (functions are callable)
print(callable(42))       # False (integers are not callable)
print(callable(str))      # True (classes are callable)
print(callable(print))    # True (built-in functions are also callable)

# Objects that define __call__ are also callable.
class Multiplier:
    def __init__(self, factor):
        self.factor = factor

    def __call__(self, x):
        return x * self.factor

triple = Multiplier(3)
print(callable(triple))  # True
print(triple(5))         # 15

# Use iter() and next() to iterate over a list manually.
fruits = ["apple", "orange", "grape"]
it = iter(fruits)

print(next(it))  # apple
print(next(it))  # orange
print(next(it))  # grape
# print(next(it))  # Raises StopIteration.

# Specifying a default value prevents an exception from being raised.
it2 = iter(fruits)
print(next(it2, "none"))  # apple
print(next(it2, "none"))  # orange
print(next(it2, "none"))  # grape
print(next(it2, "none"))  # none (no more elements)

# Two-argument form of iter(): calls the function repeatedly until the sentinel value is returned.
import random
# Simulated dice: keeps rolling until a 6 appears.
rolls = list(iter(lambda: random.randint(1, 6), 6))
print(f"Rolled {len(rolls)} time(s) before getting a 6.")

# Use hash() to check the hash value of an object.
print(hash("hello"))   # Hash value of a string (varies between runs)
print(hash(42))        # 42 (integer hash values often equal the integer itself)
print(hash((1, 2, 3))) # Tuples are hashable.
# print(hash([1, 2, 3]))  # TypeError: lists are not hashable.

Python's for loop internally calls iter() to create an iterator, then repeatedly calls next() until StopIteration is raised, at which point the loop ends. Understanding this mechanism makes it easier to grasp how generators and custom iterators work.

If you do not specify a default value for next(), calling it on an exhausted iterator will raise a StopIteration exception. When using next() outside of a loop, always specify a default value or catch the exception with a try-except block.

hash() cannot be used on mutable objects such as lists or dictionaries. Only hashable (immutable) objects can be used as dictionary keys or set members. To enable hashing in a custom class, implement the __hash__() method. For iterating over files, see open().