Python 3: Object, ID, Type

By Xah Lee. Date: . Last updated: .

Object ID

Every object has a id. it's a unique number of current session, based on memory address. 

# sample id values
print(id(3))     # 137396032
print(id(4))     # 137396048
print(id("3"))   # 3070494944
print(id(""))    # 3070685728

Object Type

Every object has a type.

print(type(4))   # <class 'int'>
print(type(4.2)) # <class 'float'>
print(type("3")) # <class 'str'>
print(type( [3, 4, 5] ))   # <class 'list'>
print(type( (3, 4, 5) ))   # <class 'tuple'>

print(type(type(4)))  # <class 'type'>

Testing If Objects Are Identical

is operator tests if two objects have the same ID. 

print(3 is 4)    # False
print(3 is 3)    # True
print("7" is "7")     # True

Object Value

Every object has a value. (here, the value means python's internal value, not what you see when you call print().)

print(id([3, 4]))  # 3069953132   (note: id may not be the same for you)
print(id([3, 7]))  # 3069953132
# from the output, you can see that [3, 4] and [3, 7] were the same object, yet the value changed

# all python object has value.
# here, “value” doesn't mean a variable's value, but internal value of python object

# 3 is a object
print(3)

# The value of the object 3 represents the math idea 3, in python's guts.
# The value of the object 3 cannot be changed.

# array is a object
print([3, 4])    # [3, 4]
# its values is a container to 2 other python objects, 3 and 4

Mutable vs Immutable Objects

Object are said to be “mutable” or “immutable”.

Example of mutable object: List.

Example of immutable object: String, Tuple.

# mutable objects (For example, lists, dictionary) may have the same value, but are not same object
print([3,4] is [3,4]) # False

aa = [3, 4]
bb = [3, 4]
print(aa is bb)    # False

# here, aa and bb are same object.
aa = [3, 4]
bb = aa
print(aa is bb)    # True

# changing aa also change bb, because they are same object
aa[0] = "x"
print(bb)   # ['x', 4]