What Happens When You Add True + True in Python?

Interview Question: True + True == ?

Python’s "+" operator is overloaded: it performs arithmetic when the operands are numbers and concatenation when they are sequences.

In  : 1 + 1
Out : 2

In  : 'abc' + '123'
Out : 'abc123'

What if the operands are Booleans? Would you think True + True == True ? Before revealing the answer, let’s review Python’s Boolean data type.

Boolean Values

The Boolean type has only two values, True and False, and is used as the basis for logical decisions in statements such as if, while, and for.

Expressions like i > 0 or objects such as 1 or 'abc' are evaluated in a Boolean context, ultimately yielding a Boolean result.

Expression return value:

In  : i = 2
In  : i > 1
Out : True

Custom objects returning Boolean values:

If a custom object implements the special method __nonzero__ (Python 3.x uses __bool__), the method is implicitly called when the object is used in a Boolean context or passed to bool().

class TestBool(object):
    def __nonzero__(self):
        print("[*] Call TestBool.__nonzero__")
        return True

if __name__ == '__main__':
    print("Invoked function bool: %s" % bool(TestBool()))
    if TestBool():
        print("TestBool() return True.")
    else:
        print("TestBool() return False.")

OUTPUT:

[*] Call TestBool.__nonzero__
Invoked function bool: True

[*] Call TestBool.__nonzero__
TestBool() return True.

Besides custom objects, all non‑empty built‑in objects are treated as True in a Boolean context, while empty ones are False.

Boolean Type Is a Special Integer Type

Boolean objects support regular arithmetic because bool is a subclass of int, inheriting many integer methods such as __eq__ and __add__.

In  : bool.__bases__
Out : (int,)

In  : True == 1
Out : True

In  : True + True
Out : 2

In  : True + 1
Out : 2

Even though True and 1 have the same value, they are different objects because their types differ; only when id, value, and type are all identical are two objects considered the same.

In  : True is 1
Out : False

In  : type(True), type(1)
Out : (bool, int)

When using augmented assignment on a Boolean, Python creates a new local variable rather than modifying the built‑in True object.

In  : type(True)
Out : bool

In  : id(True)
Out : 140735725425440

In  : True += 1
In  : True
Out : 2

In  : type(True)
Out : int

In  : id(True)
Out : 140618394261472

In Python 2.x this syntax is allowed because True and False are just variable names in the LEGB namespace; assigning to them can even swap their meanings. In Python 3.x they are reserved keywords, and attempting to assign to them raises a SyntaxError:

In [16]: if = 1
  File "<ipython-input-16-...>", line 1
    if = 1
       ^
SyntaxError: invalid syntax