How does python build its AST

[Michael Spencer]

MonkeeSage wrote:
> A quick question about how python parses a file into compiled
> bytecode. Does it parse the whole file into AST first and then compile
> the AST, or does it build and compile the AST on the fly as it reads
> expressions? (If the former case, why can't functions be called before
> their definitions?)
> 
> Thanks,
> Jordan
Python (2.5+) does parse the whole file top-to-bottom into an AST, and then 
compile the entire AST, resulting in a module code object that may contain other 
compiled code objects for the function bodies.

However, this isn't relevant to your parenthesized question, since compiling 
functions does not make them callable.  A function becomes available only once 
its def statement has been executed.  Since execution (generally) proceeds top 
to bottom, this is why functions can't be called before their definitions.

HTH
Michael