len() / type() / id()

# Returns the number of elements (length) of an object.
len(object)

# Returns the type of an object.
type(object)

# Returns the memory identity (an integer) of an object.
id(object)

# Checks whether an object is an instance of the specified type.
isinstance(object, type)

# Checks whether a class is a subclass of the specified class.
issubclass(subclass, parent_class)

sample_len_type_id.py

# Use len() to get the length of various objects.
print(len("hello"))          # 5 (number of characters)
print(len([1, 2, 3, 4]))     # 4 (number of list elements)
print(len({"a": 1, "b": 2})) # 2 (number of dictionary keys)
print(len((10, 20, 30)))     # 3 (number of tuple elements)

# Use type() to check the type of an object.
x = 42
y = "hello"
z = [1, 2, 3]
print(type(x))   # <class 'int'>
print(type(y))   # <class 'str'>
print(type(z))   # <class 'list'>

# Use id() to check the identity of an object.
a = [1, 2, 3]
b = a          # refers to the same object
c = [1, 2, 3]  # a different object
print(id(a) == id(b))  # True (same object)
print(id(a) == id(c))  # False (different objects)

# Use isinstance() to check the type of a value.
num = 3.14
print(isinstance(num, float))        # True
print(isinstance(num, (int, float))) # True (multiple types can be specified)
print(isinstance(num, str))          # False

# Use issubclass() to check the inheritance relationship between classes.
print(issubclass(bool, int))   # True (bool is a subclass of int)
print(issubclass(int, float))  # False

python3 len_type_id.py
5
4
2
3
<class 'int'>
<class 'str'>
<class 'list'>
True
False
True
True
False
True
False

len() works with all of Python's built-in sequence types, including lists, strings, tuples, dictionaries, and sets. To use it with a custom class, implement the __len__() method in that class.

type() returns the type of an object when called with one argument, and dynamically creates a new class when called with three arguments. For type comparisons, it is recommended to use isinstance(x, int) rather than type(x) == int. isinstance() is more flexible because it also matches subclasses.

id() returns a unique integer that corresponds to the object's memory address. In Python, small integers and short strings are interned (cached), so they may unexpectedly share the same ID. When you need to check object identity, using the is operator is clearer. For type conversion, see int() / float() / str() / bool().