At Least Python Got Equality Right

I’m still flabbergasted after going through the analysis of PHP == operator, posted by my infosec friend Gynvael Coldwind. Up until recently, I knew two things about PHP: (1) it tries to be weakly typed and (2) it is not a stellar example of language design. Now I can confidently assert a third one…

It’s completely insane.

However, pondering the specific case of equality checks, I realized it’s not actually uncommon for programming languages to confuse the heck out of developers with their single, double or even triple “equals”. Among the popular ones, it seems to be a rule rather than exception.

Just consider that:

• JavaScript has both == and ===, exactly like PHP does. And the former is just slightly less crazy than its PHP counterpart. For both languages, it just seems like a weak typing failure.
• In C and C++, you may easily use = (assignment) in lieu of == (equality), because the former is perfectly allowed inside conditions for if, while or for statements.
• Java is famously counterintuitive when it comes to comparing strings, requiring to use String.equals method rather than == (like in case of other fundamental data types). Many, many programmers have been bitten by that. (The fact that under certain conditions you can compare strings char-by-char with == doesn’t exactly help either).
• C# complicates stuff even more by allowing to override Equals and overload == operator. It also introduces ReferenceEquals which usually works like ==, except when the latter is overloaded. Oh, and it also has two different kinds of types (value and reference types) which by default compare in two different ways… Joy!

The list could likely go on and include most of the mainstream languages but one of them would be curiously absent: Python.

You see, Python got the == operator right:

• It tests for equality only, not identity (also known as “reference equality”). For that there is a separate is operator.
• All basic objects – not only strings, but also lists or dictionaries (hash tables) – compare by value. Hence e.g. two lists are equal if they contain equal elements in the same order, whether or not they are the same objects.
• Implicit conversions are applied judiciously. Different types of numbers (int, long, float) compare to each other just fine, but there is clear distinction between 42 (number) and "42" (string).
• You can overload == but there are no magical tricks that instantly turn your class into wannabe fundamental type (like in C#). If you really want value semantics, you need to write that yourself.

In retrospect, all of this looks like basic sanity. Getting it right two decades ago, however… That’s work of genius, streak of luck – or likely both.

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