stackfull

PYTHON STACKFULL

This small utility package provides functions to be used inside Python expressions that provide functionality like that available in stack-based languages (such as forth and postscript).

It registers a hidden variable in the current running frame, which is a plain Python list - calls to plain "push, pop, dup, retr", etc... will just push/recover elements from that list.

The intent is that whenever in an expression a value is complicated to retrieve - (or computationally extensive) - instead of having to retrieve it in a previous line and storing said value in a variable, one gets the ability to "push" the value in this implicit stack - and retrieve it in another part of the same expression::

 `result = pop().upper() if not push(get_my+expensive_value()).isdigit() else pop()`

For convenience, most functions return the valued passed to them in the first place (like the push above).

Compatibility

As of version 0.20, and introduction of the window function, stackfull is no longer Python 2.x compatible.

Available functions:

clear

Clears the stack

cleartomark

Clears the stack up to the special sentinel value
MARK - this allows for clean-up of the stack after
a block of code, preserving the older values.
If the stack is not empty, returns the last value on the
stack non destructively, else, MARK itself is returned.

discard_if_false

Removes the last value in the stack if the expression is false.
Made to be used in comprehensions, in the if part:

Ex.:
[pop().name for image in values if pop_if_false(push(image) is not None)]

dup

Dplicates the last value on the stack
It also returns the value duplicated in a non-destructive way

pop

Pops the last value from the stack

popclear

Pops the last value from the stack, and clears the stack.
This allows stackfull to be used inside generator expressions
and comprehensions, using a 'push' in the filtering expression,
and 'popclear' on the result expression.

push

Pushes a value into the stack, and returns the value itself
Along with 'pop' and 'popclear' this is the heart of
stackfull - as it allows an expensive function to be used
in a 'if' or 'while' test, and still have its value
available to use inside the defined block - without
the need of an explicit auxiliar variable

push_if_true

Returns the value itself, and Pushes a value into the stack, if it
is truthy. Otherwise does not touch the stack.
Nice to use inside  comprehensions
in the "if" part: if the expession is not True, it is never pushed, and
extraneous values don't pile up on the stack.

Ex.:
[pop().name for image in values if push_if_true(image)]

retr

Peeks the last value on the stack without consuming it

roll

Rolls the last 'items' values on the stack by
'amount' positions, changing their order.
Returns the value on the top of the stack after
the changes in a non destructive way.

stack

Retrieves the stack as an ordinary Python list
(which it actually is), allowing one to perform
extra desired operations, such as 'len' or 'insert'

window

Pre-populates a frame stack with the seed values,
and then iterates over the iterable -

This allows one to use the stack with initial values in a simple way
in a generator-expression context - like

`fib = [push(stack()[-2] + stack()[-1]) for i in window(range(2, 10), 1,1)]`

João S. O. Bueno [e-mail]

Help on module stackfull:

NAME stackfull - Stack expression utilities

DESCRIPTION These functions use a "hidden" variable in the calling code frame context to hold a stack (a Python list) with values - the simple functionality here allow one have much of the functionality of stack based languages, like Postscript and Forth conveninently available to be used in any Python expression.

(MAYBE) TODO: create a decorator which trasnform the operations here
in real operations in the Python VM stack,
by modifying the bytecode of calls to these utility functions into
actual Python VM stack operations.

FUNCTIONS clear() Clears the stack

cleartomark()
    Clears the stack up to the special sentinel value
    MARK - this allows for clean-up of the stack after
    a block of code, preserving the older values.
    If the stack is not empty, returns the last value on the
    stack non destructively, else, MARK itself is returned.

dup()
    Dplicates the last value on the stack
    It also returns the value duplicated in a non-destructive way

pop()
    Pops the last value from the stack

popclear()
    Pops the last value from the stack, and clears the stack.
    This allows stackfull to be used inside generator expressions
    and comprehensions, using a 'push' in the filtering expression,
    and 'popclear' on the result expression.

push(value)
    Pushes a value into the stack, and returns the value itself
    Along with 'pop' and 'popclear' this is the heart of
    stackfull - as it allows an expensive function to be used
    in a 'if' or 'while' test, and still have its value
    available to use inside the defined block - without
    the need of an explicit auxiliar variable

retr()
    Peeks the last value on the stack without consuming it

roll(items, amount)
    Rolls the last 'items' values on the stack by
    'amount' positions, changing their order.
    Returns the value on the top of the stack after
    the changes in a non destructive way.

stack()
    Retrieves the stack as an ordinary Python list
    (which it actually is), allowing one to perform
    extra desired operations, such as 'len' or 'insert'

DATA MARK = SN = '.stackfull'

FILE /home/gwidion/projetos/stackfull/stackfull.py