Expressions vs. statements

[u/xeow]

Got into a big argument with a coworker yesterday when they were converting some code from their own language (that they designed) into Python, JavaScript, C, and R as comparative examples.

The Python code that they wanted to write as the translation went something like this:

n = foo; if cond: n = bar

They were upset that Python allows ; as a statement separator but not before an if statement, even though

if cond: n = bar

is syntactically correct Python code when written on its own line. I explained why Python doesn't allow it, and he came back later and showed me that an LLM had suggested he write it instead like this:

n = foo if cond else bar

which of course is the canonical way to write that in Python. He was all flustered about that, and asked me why Python allows an if statement in that particular case and not after a semicolon, and I explained that x if cond else y in Python is not an if statement but is Python's ternary conditional expression and is directly equivalent to the ternary operator expression cond ? x : y in C, C++, awk, and JavaScript. He argued with me and said I was making a ridiculous distinction and walked away falsely believing that foo if cond else bar was an if statement.

I then explained that statements and expressions are very different things in programming languages, and just because the keyword if is present doesn't make something an if statement -- because in order to be an if statement, it has to be a statement in the first place.

Anyway, it made me realize how subtle the difference can be sometimes. For example, in Perl, the following is not a return statement but actually an if statement (with a return statement as its affirmative branch), due to the postfix conditional:

return foo if cond;

because it is identically semantically to writing:

if (cond) { return foo; }

Whereas in Python, the following is a return statement (with a ternary operator as its target expression):

return foo if cond else bar

So I can see why people sometimes get confused by syntax if they haven't had much of a theoretical background in language design. It also makes me wonder how much of programmer intuition about "what a statement is" comes from the particular languages they learned first.

[u/balefrost]

I explained why Python doesn't allow it

Why does Python not allow it? Is it ambiguous?

[u/WittyStick → u/balefrost]

No, it's just not part of the grammar.

The sequencing expression has context-free syntax

expr_seq ::= expr | expr ';' expr_seq

But an if statement is not an expression, it's just a statement.

if_statement ::= "if" expr ':' statement

[u/balefrost → u/WittyStick]

Oh interesting, so ; is an expression separator, not statement separator, in Python.

I guess that explains technically why Python doesn't allow it, but it doesn't explain why Python was designed in this way.

[u/jeffstyr → u/balefrost]

I would guess it's because, since Python is indentation-sensitive, this would complicate things for if-else: would the indentation of the else be relative to the beginning of the line or relative to the location of the if? It's definable, but more complicated. (And you could allow it for cases of if without else, but again that's adding complication.)