Python Internals: Understanding Python data model (I)

Python sees everything as object. Every object has an identity, value and a type. Object identity and type are invariable.

Python Data Model

Object type determines if value is mutable or otherwise.

Lifetime of object is based on reference count mechanism.

Object Container

Object containers are: list, dictionary, tuple, set. Containers keep reference (object identity) to objects. Mutability of container is based on references. The value of the referred object could be mutable.

Let’s see how they behave:

from sys import getrefcount

a = 1
b = 1

list1 = [] 
list2 = [] 

t1 = (a, b)
t2 = (a, b)

# a and b share reference to same object ID 
print "a=", id(a)
print "b=", id(b)

# Constant 1 has ref count of +2 (a and b)
print "getrefcount(1)=", getrefcount(1)

# Constant 1 has ref count of +3 now (a,b and c)
c = 1
print "getrefcount(1)=", getrefcount(1)

# Decrement the object ref count
del c
print "getrefcount(1)=", getrefcount(1)

# Default ref count of an unused new integer object is 3. But, why?
print "getrefcount(999999)=", getrefcount(999999)

print ""

# Mutable objects like list do not refer to same object ID, even
# though value of objects are same!
print "list1=", id(list1)
print "list2=", id(list2)
print "getrefcount(list1)=", getrefcount(list1)
print "getrefcount(list2)=", getrefcount(list2)

print ""

print "t1=", id(t1)
print "t2=" ,id(t2)

# Containers have default ref count of 2
print "getrefcount(t1)=", getrefcount(t1)
print "getrefcount(t2)=", getrefcount(t2)

# Changing contained object values do not modiy immutable container
a = 3
b = 10

print "t1=", id(t1)
print "t2=" ,id(t2)

# String literals are constant and are referred to like numbers
s1 = "hello"
s2 = "hello"

# s1 and s2 refer to same object
print "s1=", id(s1)
print "s2=" ,id(s2)
print "getrefcount(hello)=", getrefcount("hello")

# The first use of a literal uses 3 ref count
print "getrefcount(hello!!)=", getrefcount("hello!!")

s3 = "hello!"
print "getrefcount(hello!)=", getrefcount("hello!")