A question on Boolean Data Type

[Hudjefa]

Below are a few things I learned I could do with the Boolean Data Type

print(str(bool(3 == 3)))

I also have a fair understanding of how Boolean logic (and/or/not) works.

Before Python I was learning how to code with Microsoft's Small Basic. It also has Boolean features e.g. I can work with compound conditions using the built in and & or.

The difference between Small Basic and Python is that with Python I can access the Boolean information (True/False) as demonstrated by my 1 line code above. I know Small Basic is a very simple language and Python is well-known to be a very powerful language. I'd like to know why the Boolean information is accessible in Python? Which aspect of coding does it enhance?

P. S. The course I'm taking explicitly states we shouldn't do the following:

if 3 > 2 == True:

which is I think comparing Boolean data.

It's the same thing as

if 3 > 2:

Thank you.

[deanhystad]

There is no reason to use str() inside a print statement. Conversion of non-str to str happens automatically to all the values in a print statement.

print(3 == 3, bool(3 == 3), str(3 == 3), str(bool(3 == 3)))

Output:
True True True True

Your course is correct. You should not test if the result of a comparison is True or False. True and False should hardly ever appear in your programs. Many logical expressions in Python don't evaluate to True or False For example:

a = [1, 2, 3]
while a:
    print(a.pop(0), a)

Output:
1 [2, 3]
2 [3]
3 []

This works because an empty list in python is False when used in a logical expression and a non-empty evaluates to True. Once all numbers are removed from a, the while loop ends because an empty list is logically the same as False.

If a non-empty list is evaluates as True and True obviously evaluates as True, then this should work.

a = [1, 2, 3]
while a == True:
    print(a.pop(0), a)

But this doesn't print anything at all.

A non-empty list is true-ish, but it is not True. Both [1, 2, 3] and True are true-ish, but they are not equal.
So how does Python know when something is true-ish or false-ish? Python knows because all things in Python are instances of a class, and all classes define or inherit a method named "__bool__()" that python uses when it needs to treat a non-boolean object as a boolean object. Some things that evaluate to False when their __bool__() method is called are:
False, None, the int 0, an empty list, an empty tuple, an empty set, an empty dictionary, a zero length str (pretty much any collection type that is empty).

Things that evaluate to True when their __bool__() method is called are:
Anything that doesn't return False. Almost everything in Python except the short list above is True-ish.

[Hudjefa]

There is no reason to use str() inside a print statement. Conversion of non-str to str happens automatically to all the values in a print statement.

Didn't know that! I was confused between print functions where I had to concatenate non-string values with strings:

print("You ate " + str(number_pies) + " pies"

I will save the rest of your post for read later. Once I get to know what a list is.

So you're saying the Boolean data for a condition is hardly ever accessed?

Do you think the following line will ever be useful in a code?

name_is_not_null = bool(name)
if name_is_not__null:

What's the difference between the above and

length_name = len(name)

if length_name = 0:

[deanhystad]

A better way to do this:

print("You ate " + str(number_pies) + " pies")

Is to use a format string.

print(f"You ate {number_pies} pies")

I'm saying that True and False are hardly ever used. Boolean values are used all the time. Any comparison (==, >, <, >=, <=, !=) returns a boolean. You saw that in my previous post where I printed the result of 3 == 3 and it printed True. But the value of a logical expression does not have to equal True or False. If the result of a logical expression is not True or False, python uses __bool__() to get True or False.

This is not good code:

name_is_not_null = bool(name)
if name_is_not__null:

There is no reason for you to call bool. The code should be written as:

if name:

You should never write anything like this either:

length_name = len(name)
 
if length_name = 0:

Instead writing as:

if len(name) == 0:
# or
if not name:

[Gribouillis]

As deanhystad said above, some conversions occur implicitly, for example

print(expression)

is equivalent to

print(str(expression))

and

if expression:
    ...

is equivalent to

if bool(expression):
    ...

but in some cases, there is no implicit conversion, for example

x and y

does not necessarily return a boolean. In fact it returns y if bool(x) is True, otherwise it returns x. For example

>>> 4 and 7
7
>>> 4 and 0.0
0.0
>>> [] and [1, 3]
[]

So it is the same as

y if x else x

In the same way

x or y

is equivalent to

x if x else y

it returns x if bool(x) is True, otherwise it returns y.

[Hudjefa]

Aah I see. It's more complex than I thought it was. My lessons were short 5 minute max videos. I should've paid attention to the course name: Intro to ...

Gracias.