Operations and Operators

Previous

Home

Next

Truth Value Testing

Python Docs - Truth Value Testing

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:

• None
• False
• zero of any numeric type, for example, 0, 0.0, 0j.
• any empty sequence, for example, '', (), [].
• any empty mapping, for example, {}.
• instances of user-defined classes, if the class defines a __bool__() or __len__() method, when that method returns the integer zero or bool value False. [1]

All other values are considered true — so objects of many types are always true.

Operations and built-in functions that have a Boolean result always return 0 or False for false and 1 or True for true, unless otherwise stated. (Important exception: the Boolean operations or and and always return one of their operands.)

Boolean Operations

and, or, not

Python Docs - Boolean Operations

These are the Boolean operations, ordered by ascending priority:

Notes:

1. This is a short-circuit operator, so it only evaluates the second argument if the first one is False.
2. This is a short-circuit operator, so it only evaluates the second argument if the first one is True.
3. not has a lower priority than non-Boolean operators, so not a == b is interpreted as not (a == b), and a == not b is a syntax error.

Examples:

not:

>>> not True
False
>>> not False
True

and:

>>> True and False    # Short-circuited at first argument.
False
>>> False and True    # Second argument is evaluated.
False
>>> True and True     # Second argument is evaluated.
True

or:

>>> True or False    # Short-circuited at first argument.
True
>>> False or True    # Second argument is evaluated.
True
>>> False or False   # Second argument is evaluated.
False

Parenthesis for boolean operations

Just as in math, parenthsis can be used to override order of operations:

>>> not True or True
True
>>> not (True or True)
False

>>> True or False and False
True
>>> (True or False) and False
False

Comparisons

Python Docs - Comparisions

There are eight comparison operations in Python. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; for example, x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).

This table summarizes the comparison operations:

Objects of different types, except different numeric types, never compare equal. Furthermore, some types (for example, function objects) support only a degenerate notion of comparison where any two objects of that type are unequal. The <, <=, > and >= operators will raise a TypeError exception when comparing a complex number with another built-in numeric type, when the objects are of different types that cannot be compared, or in other cases where there is no defined ordering.

Non-identical instances of a class normally compare as non-equal unless the class defines the __eq__() method.

Instances of a class cannot be ordered with respect to other instances of the same class, or other types of object, unless the class defines enough of the methods __lt__(), __le__(), __gt__(), and __ge__() (in general, __lt__() and __eq__() are sufficient, if you want the conventional meanings of the comparison operators).

The behavior of the is and is not operators cannot be customized; also they can be applied to any two objects and never raise an exception.

TODO

• Chained comparisons w < x < y > z
• Equality comparison is vs ==

Numeric Operations

Python Docs - Numeric Operations

Python fully supports mixed arithmetic: when a binary arithmetic operator has operands of different numeric types, the operand with the “narrower” type is widened to that of the other, where integer is narrower than floating point, which is narrower than complex. Comparisons between numbers of mixed type use the same rule. [2] The constructors int(), float(), and complex() can be used to produce numbers of a specific type.

All numeric types (except complex) support the following operations, sorted by ascending priority (all numeric operations have a higher priority than comparison operations):

Notes:

1. Also referred to as integer division. The resultant value is a whole integer, though the result’s type is not necessarily int. The result is always rounded towards minus infinity: 1//2 is 0, (-1)//2 is -1, 1//(-2) is -1, and (-1)//(-2) is 0.

2. Not for complex numbers. Instead convert to floats using abs() if appropriate.

3. Conversion from floating point to integer may round or truncate as in C; see functions math.floor() and math.ceil() for well- defined conversions.

4. float also accepts the strings “nan” and “inf” with an optional prefix “+” or “-” for Not a Number (NaN) and positive or negative infinity.

5. Python defines pow(0, 0) and 0 ** 0 to be 1, as is common for programming languages.

6. The numeric literals accepted include the digits 0 to 9 or any Unicode equivalent (code points with the Nd property).

   See Unicode Derived Numeric Type for a complete list of code points with the Nd property.

TODO

• Add excercises

Using interactive mode as a calculator

TODO

• Add info about using _ for previous output in interactive mode.

We shall be covering a few more in-built types next.