pySCG: Adding explanation of the 'is' operator to CWE-595

[s19110]

Addresses issue #714

[myteron]

provided example01.py code in the md for the obsevered issues around numbers and strings to explain memory optimistation related isses.

[myteron]

Suggstion for first sentence:

Prevent unexpected results by knowing what comparisment operators do such as == and is actually do.

After the existing first sentence we shall also explain:

Python falls back to comparing objects id() if the `eq implementation is missing for a custom class.

[myteron]

Be aware of Python's memory optimization for strings and numbers as demonstrated in example01.py code.
Python tries to avoid allocating more memory for the same string. Numbers -5 to 256 are so frequently used that they are pre-reserved.

# SPDX-FileCopyrightText: OpenSSF project contributors
# SPDX-License-Identifier: MIT
""" Code Example """

print("-" * 10 + "Memory optimization with strings" + 10 * "-")
a = "foobar"
b = "foobar"
c = ''.join(["foo", "bar"])
print(f"a is b: {a} is {b}?", a is b)
print(f"a is c: {a} is {c}?", a is c)
print(f"a == c: {a} == {c}?", a == c)
print(f"size? len(a)={len(a)} len(b)={len(b)} len(c)={len(c)}")

print("-" * 10 + "Memory optimization with numbers" + 10 * "-")
a = b = 256 
print (f"{a} is {b}?", a is b)
a = b = 257
print (f"{a} is {b}?", a is b)

print("-" * 10 + "Memory optimization with numbers in a loop" + 10 * "-")
a = b = 255
while(a is b):
    a += 1 
    b += 1
    print (f"{a} is {b}?", a is b)

The example01.py code output:

a is b: foobar is foobar? True
a is c: foobar is foobar? False
a == c: foobar == foobar? True
size? len(a)=6 len(b)=6 len(c)=6
----------Memory optimisation with numbers----------
256 is 256? True
257 is 257? True
----------Memory optimisation with numbers in a loop----------
256 is 256? True
257 is 257? False

The first print statement illustrates Python's memory optimization for strings.
See id(a) and id(b) printing the same object number compared to id{c} printing a different id for t he same string.

The example in the middle creates same object number as a = b = 257 tells python that it can used the same memory.

The last print statements in example01.py illustrate Python's memory optimization for numbers between -5 to 256. Python needs to allocate new objects for numbers greater then 256. Note that the behavior of Python for numbers between -5 and 256 change depending on how its run. Following code changes behavior when run via interactive Python shell or in a file:

example02.py:

a = 256
b = 256
print(a is b)
a = 257
b = 257
print(a is b)

Using an interactive Python shell will print True and False while running the code in a python script will print True and True as the runtime has optimized the code to preserve memory.