By default, statements in Python are executed sequentially.
*Branching* statements are used to
break this sequential pattern.

Sometimes you want to perform certain operations only in some cases. This is called a

*conditional branch*.Sometimes you need to perform some operations repeatedly. This is called

*looping*.

Before we look at how Python does conditional branching, we need to look at Python's Boolean type.

Boolean algebra is the mathematics of true/false
decisions. Python's `bool`

type has only
two values: `True`

and `False`

.

A typical use of Boolean algebra is in comparing two
values. In Python, the expression

is * x* <

`y`

`True`

if `x`

is less than `y`

, `False`

otherwise.
>>> 2 < 5 True >>> 2 < 2 False >>> 2 < 0 False

Here are the six comparison operators:

Math symbol | Python | Meaning |
---|---|---|

< | < | Less than |

≤ | <= | Less than or equal to |

> | > | Greater than |

≥ | >= | Greater than or equal to |

≡ | == | Equal to |

≠ | != | Not equal to |

The operator that compares for equality is “`==`

”. (The “`=`

”
symbol is not an operator: it is used only in the
assignment statement.)

Here are some more examples:

>>> 2 <= 5 True >>> 2 <= 2 True >>> 2 <= 0 False >>> 4.9 > 5 False >>> 4.9 > 4.8 True >>> (2-1)==1 True >>> 4*3 != 12 False

Python has a function `cmp(`

that compares two values and returns:
* x*,

`y`

Zero, if

and`x`

are equal.`y`

A negative number if

.<`x`

`y`

A positive number if

.>`x`

`y`

>>> cmp(2,5) -1 >>> cmp(2,2) 0 >>> cmp(2,0) 1

The function `bool(`

converts any value * x*)

`x`

to a Boolean value. The values
in this list are considered `False`

; any
other value is considered `True`

:
Any numeric zero:

`0`

,`0L`

, or`0.0`

.Any empty sequence:

`""`

(an empty string),`[]`

(an empty list),`()`

(an empty tuple).`{}`

(an empty dictionary).The special unique value

`None`

.

>>> print bool(0), bool(0L), bool(0.0), bool(''), bool([]), bool(()) False False False False False False >>> print bool({}), bool(None) False False >>> print bool(1), bool(98.6), bool('Ni!'), bool([43, "hike"]) True True True True