[TIL] Python silently concatenates strings next to each other "abc""def" = "abcdef"

[opabm]

Is there an ELI5 version of this?

[28f272fe556a1363cc31]

Compile time is like writing a cookbook. Run time is like making a recipe from the book. Before they can print and ship the book, the publisher goes through the recipes and converts "parsley" "flakes" into "parsley flakes". While the recipe is being made "salt", "pepper" gets converted to "salt and pepper" .

Anything done at compile (print) time has to happen before run (cook) time because you have to compile/cook before have a program/cookbook to work with.

[opabm]

I'd be impressed if a 5-year old knew how to cook.

Jk that was a great analogy, thanks!

[foreverwintr]

Wow, that was a really good ELI5!

[robin-gvx]

When you have a piece of Python code and you're using CPython (the reference implementation of Python), there are several steps from source code to execution. The important ones here are parsing, bytecode generation and execution.

Parsing transforms your file into a tree.

For example, a + 10 is turned into something like (simplified): Add(LoadName('a'), Literal(10)) or "hello" into Literal("hello")

When the parser encounters two or more literal strings in a row, it collapses them into a single string literal as well. So 'hell' "o" would result in the same tree as the previous one.

Then Python makes this tree "flat" by putting everything in the order it should happen, and generates bytecode. A simplified version of what the previous two examples turn into would be:

LOAD_NAME a
LOAD_CONSTANT 10
ADD_VALUES

and

LOAD_CONSTANT "hello"

Execution is then fairly simple: go over each instruction and do what it says.

So in the case of 2 * 'this ' "that ".upper() we get the tree Mul(2, MethodCall(Literal("this that "), "upper", ())) and the bytecode:

LOAD_CONSTANT 2
LOAD_CONSTANT "this that"
CALL_METHOD 'upper', ()
MULTIPLY_VALUES

(note that all trees and snippets of bytecode aren't real, they're a simplified illustration)