CODEWITHSUNDEEP
pythonpython-3.x
TheValars
TheValars

Reputation: 281

What is this condition trying to compare?

I am currently learning python and encountered this function.

def min(root, min_t): # min_t is the initially the value of root
    if not root:
        return min_t

    if root.key < min_t:
        min_t = root.key

    min_t = min(root.left, min_t)
    min_t = min(root.right, min_t)

    return min_t

I'm quite confused to what it means by "if not root". What condition is it trying to give? if "root" is not what? What comparison is "if" trying to make?

UPDATE: Root is a binary tree which has the subtrees self.left and self.right. In can also be None. This is the case if the node is a leaf.

Upvotes: 1

Views: 80

Answers (3)

jonrsharpe
jonrsharpe

Reputation: 122086

In Python, if expr: really means if bool(expr):, i.e. it evaluates whatever expression is given to it then converts the result to a boolean (in many cases, of course, it's already a boolean, e.g. x > y will be True or False*).

The built-in types have rules for boolean evaluation; in short, empty containers and sequences (dict, list, tuple, set, str, etc.) plus zero numerical values (0, 0.0, 0j, etc.) and None evaluate False-y, anything else is considered True-y. User-implemented types usually follow this convention.

This lets if root be a handy shortcut for e.g. if root != 0 or if len(root) > 0, depending on what type of object root is expected to be. Note that None is usually, per the style guide, tested with if name is not None; this lets you easily distinguish between None and other False-y values.

* Unless the "magic methods" are implemented in a non-standard way

Upvotes: 1

mkrieger1
mkrieger1

Reputation: 23255

The Python language reference on boolean operations explains this:

In the context of Boolean operations, and also when expressions are used by control flow statements, the following values are interpreted as false: False, None, numeric zero of all types, and empty strings and containers (including strings, tuples, lists, dictionaries, sets and frozensets). All other values are interpreted as true. (See the __nonzero__() special method for a way to change this.)

In your case, since root seems to be an instance of a user-defined type (it has left and right members, which no built-in type has), it either has the __nonzero__ method implemented, or None or a similar "empty" object is passed in to signal that there is no "left" and "right".

Upvotes: 1

rafaelc
rafaelc

Reputation: 59274

In python, you dont necessarily have to compare something to something else.

if root checks, by default, if the variable root has a content, or if it is an empty variable. Let me explain you via examples:

if ""
if 0
if None
if []
if {}
if ()

would all return False

While any other values would return True.

Upvotes: 3

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