Python internals

[Terry Reedy]

Peter Anderson wrote:
> Hi!  I am slowly teaching myself Python.  I was reading David Beazley's 
> excellent book "Python - Essential Reference"; in particular about 
> variables.  Let me quote:
> 
> "Python is a dynamically typed language in which names can represent 
> values of different types during the execution of a program. In fact the 
> names used in the program are really just labels for various quantities 
> and objects. The assignment operator simply creates an association 
> between a name and a value. This is different from C, for example, in 
> which a name (variable) represents a fixed size and location in memory..."
> 
> As an old mainframe programmer, I understand the way C does things with 
> variable but this text got me wondering how Python handles this 
> "association" between variable name and value at the lower level.  Is it 
> like a fifo list?

No.  Names are associated with (references to) objects in a namespace. 
The details are implementation specific.  For CPython, the references 
are addresses stored in an array.  If the set of names is dynamic, names 
are converted to indexes by hashing during execution and the array of 
references is expanded before it gets full.  If the set of names is 
static, as is usual for the local namespace of functions, an 
optimization converts the names to indexes (in a constant length array) 
during compilation.  (On function startup, slots corresponding to 
non-parameter local vars are, I expect, initialized to invalid reference 
values to detect 'use before assignment' bugs.)

Terry Jan Reedy