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!
=
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 likeif( a = (b+c) )
would not assign a value toa
, 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
Imagine saying these operators out loud.
is “is greater than”
So it makes sense to use
=
as “is greater than or equal to”You’d think
=
would be “is equal to”, but it’s already used for “set equal to” (i.e., assignment).So what symbol do we use for “is equal to”? The symbol used in many programming languages is
==
, so Python chose to follow that convention.It’s worth noting that there are other languages that use
=
as “is equal to”, and use something else for assignment (like:=
for example). It just comes down to the history of the language and what conventions the original authors decided to use.It would be confusing and weird if “=” did different things depending on the context.
= is the assignment operator
== is the comparison operator.
the others using = only is probably just to keep things short, and the fact that the context is a lot clearer with another character like < next to the =
Pascal uses
=
for comparison (and:=
for assignment), which confused the fuck out of me when I switched to C.Some people in mathematics use := to assign functions, like f(x) := x^2; then when evaluating the function you use f(2) = 4, because it can be ser as a “true” comparison
I’ve never seen that, even in university, and it would be equally as confusing without explanation.
I only remember two of my professors using it, and I has to ask the first one what that mean and explain to my classmates on the second one.
It’s all convention coming from older programming languages, particularly C, which comes from programmers wanting shorthand for things like “BRANCH_EQUAL $1 $2 $3” which is shorthand for some binary code.
Python has changed the logical and and or operators to be
and
andor
instead of&&
and||
.It’s a convention set by early programming languages.
In most C-like languages,
if (a = b)...
is also a valid comparison. The=
(assignment) operation returns the assigned value as a result, which is then converted to a boolean value by theif
expression. Consider this Javascript code:let a = b = 1
- It first declares the
b
variable and assigns it the value of the expression1
, which is one. - It returns the result of the expression
b = 1
, which is the assigned value, which is1
. - It declares the
a
variable and assigns the previously returned value, which is1
.
Another example:
let a = 1 let b = 2 let c = 3 console.log(a == b) // prints "false" because the comparison is false console.log(a = b) // prints 2 because the expression returns the value of the assignment, which is 'b', which is 2 // Using this in an 'if' statement: if (b = c) { // the result of the assignment is 3, which is converted to a boolean true console.log("what") }
You can’t do the same in Python (it will fail with a syntax error), but it’s better to adhere to convention because it doesn’t hurt anyone, but going against it might confuse programmers who have greater experience with another language. Like I was when I switched from Pascal (which uses
=
for comparison and:=
for assignment) to C.With python you can use the := to assign and return new value.
Walrus operator my beloved
- It first declares the
It’s still a mathematic operator. There’s an entire field of math dedicated to comparisons, and it’s featured heavily in most programming languages. Computer science is a field dedicated to the application of math
If I were to speculate, I’d say it came from the == operator (Boolean equality comparison) and then later, when that was extended to include Boolean less-than-or-equal and greater-than-or-equal, the decision was made to keep them 2 characters long. Either because it was visually cleaner, or just because programmers love being lazy (read: efficient)
Pretty sure this is directly inspired by C so I would guess Guido van Rossum (the author of Python) just used what was already common back then. As in,
=
is assignment operator and==
is equality/comparison operator.https://en.cppreference.com/w/c/language/operator_comparison
<=
is already no mathematical assignment operator, but a comparison operator. Thus there is no need to define e.g.<==
for comparing two values.I get your point of view, and haven’t thought about it that way before.
As for why it’s like that, my best guess is the sentence I wrote above. Your proposal totally makes sense.