Understanding when to use is, ==, is not, and not

1. is vs ==

is

• Purpose: Checks identity.
• It determines whether two variables refer to the same object in memory.
• Used primarily for checking singleton objects (None, True, False), or ensuring two variables point to the exact same instance.

Example:

a = [1, 2, 3]
b = a
c = [1, 2, 3]

print(a is b)  # True, because `b` refers to the same object as `a`.
print(a is c)  # False, because `c` is a different object, even if it has the same value.

==

• Purpose: Checks equality.
• It determines whether two variables have the same value, regardless of whether they refer to the same object.

Example:

pythonCopy codeprint(a == b)  # True, values are the same.
print(a == c)  # True, values are the same.

When to Use:

• is: Use when you need to ensure two variables reference the exact same object.
  • Example: if a is None: (checks for identity with the None singleton).
• ==: Use when you need to compare values.
  • Example: if content_type == "JSON": (checks for value equality).

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2. is not vs not

is not

• Purpose: Checks identity negation.
• It determines whether two variables do not refer to the same object.

Example:

x = None

if x is not None:  # Checks if `x` is not the `None` singleton.
    print("x is not None")

not

• Purpose: Negates a boolean expression.
• Converts True to False and False to True.

Example:

x = False

if not x:  # Equivalent to `if x == False:`
    print("x is False or evaluates to False")

y = [1, 2, 3]

if not y:  # Evaluates to False because the list is not empty.
    print("List is empty")

When to Use:

• is not: Use for identity negation, especially for checking if a variable is notNone, True, or False.
  • Example: if x is not None:
• not: Use to negate the truthiness of an expression.
  • Example: if not x: (to check if x is falsy).

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Comparison Chart

Operator	Checks	When to Use
is	Identity: same object in memory	For checking singletons like None, or ensuring two variables reference the same object.
==	Equality: same value	For comparing values of variables, like strings, numbers, or objects with the same content.
is not	Identity negation: not the same object	For ensuring a variable does not reference a specific singleton or object.
not	Negates the truthiness of an expression	For negating conditions or checking falsy values (None, False, empty containers, etc.).

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Examples of Usage

Checking for None

x = None

if x is None:  # Correct, checks identity with `None`.
    print("x is None")

if x == None:  # Works, but less Pythonic. Avoid in practice.
    print("x equals None")

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Comparing Objects

a = [1, 2, 3]
b = a
c = [1, 2, 3]

if a is b:  # True, `b` is the same object as `a`.
    print("a and b are the same object")

if a == c:  # True, `a` and `c` have the same value.
    print("a and c have the same value")

if a is not c:  # True, `a` and `c` are different objects.
    print("a and c are not the same object")

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Negating a Condition

x = False

if not x:  # Equivalent to `if x == False:`
    print("x is False")

y = []

if not y:  # Checks for an empty list (falsy value).
    print("y is an empty list")

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Key Notes

1. Identity vs Equality:
   • Use is and is not for identity checks (e.g., None, True, False).
   • Use == and != for comparing values.
2. Negations:
   • Use not to invert the truthiness of any expression.
   • Use is not for negating identity checks.
3. Falsy Values:
   • In Python, None, False, 0, "", [], {}, and set() are considered falsy. Use not to check these effectively.