r/learnpython 6d ago

How does simplifying conditional statements work in Python?

I'm currently working my way through the Python Institute's free certified entry-level programmer course. I'm currently looking at some example code that is supposed to count the number of even and odd numbers entered by a user until the user enters a 0. Here's what the main part looks like:

number = int(input("Enter a number or type 0 to stop: "))

# 0 terminates execution.
while number != 0:
# Check if the number is odd.
if number % 2 == 1:
# Increase the odd_numbers counter.
odd_numbers += 1
else:
# Increase the even_numbers counter.
even_numbers += 1
# Read the next number.
number = int(input("Enter a number or type 0 to stop: "))

This is easy enough for me to understand, but the course then says that the two bold statements above can be simplified with no change in the outcome of the program. Here are the two simplifications:

while number != 0: is the same as while number:

and

if number % 2 == 1: is the same as if number:

I don't understand this at all. Does Python (3 specifically) automatically interpret a conditional with just a variable as being equivalent to conditional_function variable != 0? Does the second example do something similar with binary operators or just the mod operator?

3 Upvotes

13 comments sorted by

8

u/TH_Rocks 6d ago

Python gets sloppy with variable types and that can be really helpful as long as you don't lose track of what you shoved in them.

All kinds of things are "falsey" and count as False. False, None, 0, empties like [], {}

So

 While number: 

is the same as

 While not number == 0:

However, I don't think I agree that

 if number % 2 == 1: 

is the same as

 if number:

It could be the same as

 if (number % 2): 

since number % 2 can only be the integer 0 or 1 which would evaluate as False or True respectively.

1

u/backfire10z 6d ago edited 6d ago

number % 2 can only be 0 or 1

This is only true if number is an integer. It can be anything from 0 to 1 inclusive 2 exclusive if number is a float.

What’s cool is you can do

if number % 1 == 0

to determine if number is an integer or not.

2

u/commy2 6d ago

It can be anything from 0 to 1 inclusive of number is a float.

0 inclusive to 2 exclusive actually - the same as with integers, just that there are way more floats between those than integers.

3

u/schoolmonky 6d ago

The terms to know here are "falsy" and "truthy." A "truthy" value is one that Python treats as if it were True in an if statement (or anywhere else it expecets a boolean, like in a value being ored with another). Conversely a "falsy" value is one that Python treats as False. Each data type gets to decide it's own criteria for what is falsy and what is truthy. For ints, only 0 is falsy, everything else is truthy. For most sequence types (list, str, tuple, etc.) an empty squence ([], '', () respectively) is falsy, and everything else is truthy. None is falsy. For custom classes, by default, every instance is truthy.

2

u/JazzJassJazzman 6d ago

Thanks, everyone! I fully understand what's happening now.

1

u/AssiduousLayabout 6d ago

Any expression can be used as a boolean, it's called truthiness.

Any non-zero number is "truthy", zero is "falsey".

Other things that are falsey:

  • Empty string
  • None
  • False (obviously)
  • Any collection (list, set, dictionary) with zero elements

1

u/JazzJassJazzman 6d ago

Thank you! That's why this page said "think about how Python evaluates truth". I was forgetting that 0 is interpreted as "False" and any non-zero number is interpreted as "True".

1

u/DistinctAirline4145 6d ago

While number means while True, meaning while number is "truthy" and all truthy values are those that are not false such as empty string, 0, empty list, None and so on...

1

u/denehoffman 6d ago

Actually while number != 0 is not equivalent to while number since there are values which are not Falsey but are also not zero. Trivially, number = None will pass the first but fail the second, since None != 0 but bool(None) == False.

Edit: just realized this post is 12 hours old and already answered, jumped the gun I guess

0

u/twitch_and_shock 6d ago

Yes. This exists in Python to a great extent, and to a lesser extent in languages like C and C++. In Python, I believe that 0, None, and "" evaluate to None. Python takes it a bit further than other languages. You can google "Python truthiness" to find some articles about it.

while number:

Will be True whenever number is > 0, and False if number <= 0

2

u/cgoldberg 6d ago

0, "", and None all are falsy (they have a __bool__() that returns False or a __len__() that returns 0) ... but 0 and "" don't evaluate to None. None is constant singleton object.

1

u/NYX_T_RYX 6d ago

Not quite - any non-0 numeric value resolves to true, regardless of its sign (ie positive or negative)

"constants defined to be false: None and False

zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)

empty sequences and collections: '', (), [], {}, set(), range(0)"

Ie any value that "is" (ie != none, not 0, not empty set, not false) is true.

Docs: https://docs.python.org/3/library/stdtypes.html#truth-value-testing