How do the for / while / print *things* work in python?

Question

What i mean is, how is the syntax defined, ie how can i make my own constructs like these?

I realise in a lot of languages, things like this will be built into the compiler / spec, and so it's dealt with by the compiler (at least that how i understand it to work).

But with python, everything i've come across so far has been accessible to the programmer, and so you more or less have the freedom to do whatever you want.

How would i go about writing my own version of for or while ? Is it even possible?

I don't have any actual application for this, so the answer to any WHY?! questions is just "because why not?" or "curiosity".

Answer 1

No, you can't, not from within Python. You can't add new syntax to the language. (You'd have to modify the source code of Python itself to make your own custom version of Python.)

Note that the iterator protocol allows you to define objects that can be used with for in a custom way, which covers a lot of the possible use cases of writing your own iteration syntax.

Answer 2

You can't make equivalent constructs. for , while , if etc. are statements , and they are built into the language with their own specific syntax. There are languages that do allow this sort of thing though (to some degree), such as Scala .

Answer 3

while , print , for etc. are keywords. That means they are parsed by the python parser whilst reading the code, stripped any redundant characters and result in tokens. Afterwards a lexer takes those tokens as input and builds a program tree which is then excuted by the interpreter. Said so, those constructs are used only as syntactic sugar for underlying lexical machinery and as such are not visible from inside the code.

Answer 4

Well, you have a couple of options for creating your own syntax:

1. Write a higher-order function, like map or reduce .

2. Modify python at the C level. This is, as you might expect, relatively easy as compared with fiddling with many other languages. See this article for an example: http://eli.thegreenplace.net/2010/06/30/python-internals-adding-a-new-statement-to-python/

3. Fake it using the debug facilities, or the encodings facility. See this code: http://entrian.com/goto/download.html and http://timhatch.com/projects/pybraces/

4. Use a preprocessor. Here's one project that tries to make this easy: http://www.fiber-space.de/langscape/doc/index.html

5. Use of the python facilities built in to achieve a similar effect (decorators, metaclasses, and the like).

Obviously, none of this is quite what you're looking for, but python, unlike smalltalk or lisp, isn't (necessarily) programmed in itself and guarantees to expose its own underlying execution and parsing mechanisms at runtime.