Python @programming.dev EveryMuffinIsNowEncrypted @lemmy.blahaj.zone 2w ago

Confused about logical operators (i.e. "comparison operators", or "boolean operators") in Python

Note: The attached image is a screenshot of page 31 of Dr. Charles Severance's book, Python for Everybody: Exploring Data Using Python 3 (2024-01-01 Revision).

I thought = was a mathematical operator, not a logical operator; why does Python use

>= instead of >==, or <= instead of <==, or != instead of !==?

Thanks in advance for any clarification. I would have posted this in the help forums of FreeCodeCamp, but I wasn't sure if this question was too.......unspecified(?) for that domain.

Cheers!

Edit: I think I get it now! Thanks so much to everyone for helping, and @[email protected] and @[email protected] in particular! ^_^

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33 comments

>= and <= match the mathematical operators. The question you want to ask is why doesn't it use = for equality, and the answer is that = is already used for assignment (inherited from C among other languages).

In theory a language could use = for assignment and equality but it might be a bit confusing and error prone. Maybe not though. Someone try it and report back.
- I've written code before in some hardware-specific languages before (I think it was for programming a stepper motor or something?) that used = for both assignment and comparison. If I recall correctly, the language was vaguely C-like, but assignment was not permitted in the context of a comparison. So something like if( a = (b+c) ) would not assign a value to a, it would just do the comparison.
- Rust does an interesting thing in this regard. It does still have == for checking if two values are equal, but well, it actually doesn't have a traditional assignment operator. Instead, it has a unification operator, which programmers usually call "pattern matching".
  
  And then you can use pattern matching for what's effectively an assignment and to some degree also for equivalence comparison.
  See a few examples here: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1268682eb8642af925db9a499a6d587a
  
  It does still have a traditional assignment operator. You can assign values to mutable variables.
  
  Also I would say let-binds are still pretty much assignment; they just support destructuring. Plenty of languages support that to some extent (JavaScript for example) and you wouldn't say they don't have assignment.
  
  I don't think it affects the ability to overload = anyway. I think there aren't any situations in Rust where it would be ambiguous which one you meant. Certainly none of the examples you gave compile with both = and ==. Maybe there's some obscure case we haven't thought of.
  
  This reminds me on the niche tool in Mathematica I've been using, which has four different assignment oparators for that purpose.
- I think what I'm most confused about is I cannot for the life of me seem to wrap my head around the difference between "assignment" and "equality". They seem exactly the same to me: when a variable is assigned a value, it's equal to that value now.
  
  Even if I were write the program
  
  x = 20 print(x*2)
  
  it would still print 40. Because x is equal to 20. Because it was assigned the value of 20.
  
  Hell, I've even heard Dr. Severance say "equal to" in this context in earlier videos multiple times.
  
  They seem exactly the same to me: when a variable is assigned a value, it’s equal to that value now.
  
  Yeah it's confusing because in maths they are the same and use the same symbol but they are 100% not the same in programming, yet they confusingly used the same symbol. In fact they even used the mathematical equality symbol (=) for the thing that is least like equality (i.e. assignment).
  
  To be fair not all languages made that mistake. There are a fair few where assignment is like
  
  x := 20
  
  Or
  
  x <- 20
  
  which is probably the most logical option because it really conveys the "store 20 in x" meaning.
  
  Anyway on to your actual question... They definitely aren't the same in programming. Probably the simplest way to think of it is that assignment is a command: make these things equal! and equality is a question: are these things equal?
  
  So for example equality will never mutate it's arguments. x == y will never change x or y because you're just asking "are they equal?". The value of that equality expression is a bool (true or false) so you can do something like:
  
  a = (x == y)
  
  x == y asks if they are equal and becomes a bool with the answer, and then the = stores that answer inside a.
  
  In contrast = always mutates something. You can do this:
  
  a = 3 a = 4 print(a)
  
  And it will print 4. If you do this:
  
  a = 3 a == 4 print(a)
  
  It will (if the language doesn't complain at you for this mistake) print 3 because the == doesn't actually change a.
  
  Ohhhhh! I think I get it now!
  
  So == means "equals" and is a declaration of the state of things, while = means "assigned the value of` and is a command toward a certain state of things. A description vs an action. An observation of a thing as opposed to effecting that thing.
  
  Is that about right?
  
  That's exactly it. Some languages (e.g. Rust) make it even more clear¹, by following math notation for assignment even closer:
  
  let x = 5;
  
  ¹ simplified Rust a little bit, there's a bit more nuance
  
  Thanks so much to you and @[email protected] for helping! This has been driving me crazy for like 3-4 weeks now! >_<
  
  Well == is a question or a query rather than a declaration of the state of things because it isn't necessarily true.
  
  You can write
  
  a = (3 == 4)
  
  which is perfectly valid code; it will just set a to be false, because the answer to the question "does 3 equal 4?" is no.
  
  I think you've got it anyway.
  
  How would you check two variables have equal values without changing the value of one otherwise?
  
  Assignment you are assigning a value to the left side. Equality you are checking if the left and right are equal.
  
  It's "set equal to" Vs "is equal to" one is an operation the other is a condition.
  
  ah, but consider: a+b=2
  (perfectly valid in metafont!)
  
  I apologize; I do not know what "metafont" is...
  
  programming language; tex's sibling. you define a system of equations that it solves

33 comments