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id(), type(), isinstance()

These built-ins support introspection, allowing programs to examine objects at runtime.

id()

Returns a unique identifier for an object during its lifetime:

x = []
id(x)

Conceptually corresponds to a memory address.

type()

Returns the type of an object:

type(3)        # <class 'int'>
type([1, 2])   # <class 'list'>

isinstance()

Checks whether an object is an instance of a type:

isinstance(3, int)
isinstance(True, int)   # True

Supports tuples of types:

isinstance(x, (int, float))

dir()

Returns a list of names (attributes and methods) of an object:

name = "hello"
print(dir(name))  # List of string methods

Without arguments, returns names in current scope:

x = 10
print(dir())  # Includes 'x'

Useful for exploration and debugging.

help()

Displays documentation for an object:

help(str.lower)
help(len)

Shows the docstring and signature. In interactive sessions, provides paginated output.

del

Deletes references to objects:

# Delete a variable
x = 10
del x
# x is no longer defined

# Delete from a list
my_list = [1, 2, 3]
del my_list[1]  # [1, 3]

# Delete from a dict
my_dict = {'a': 1, 'b': 2}
del my_dict['a']  # {'b': 2}

# Delete an attribute
del obj.attribute

Memory is freed by the garbage collector when no references remain.

Best practices

  • Use isinstance() instead of type(x) == T.
  • Avoid excessive type checking; prefer polymorphism.

Key takeaways

  • id() returns object identity (memory address)
  • type() returns object type
  • isinstance() is the correct type-checking tool
  • dir() lists available attributes and methods
  • help() displays documentation
  • del removes references (not the object itself)