the stupid postfix evaluator
Project description
postfixcalc
Simple and stupid infix to postfix converter and evaluator.
How does it work
The algorithm is very simple and straightforward
from postfixcalc.pyeval import evaluate
from postfixcalc.parser import (
extract_nums_and_ops,
flatten_nodes,
infix_to_postfix,
make_num,
parse,
relistexpression,
)
evaluate(
infix_to_postfix(
make_num(
relistexpression(
flatten_nodes(
extract_nums_and_ops(
parse('(-1) ^ 2')
),
),
),
),
),
)
We should trace from bottom to top:
- parse the expression using
ast.parse
function. This function will parse the expression based on Python grammar and math op precedence. - extract numbers, and operators outta parsed expression
- the extracted list contains many nested lists and tuples, so we flatten most of them
- we generate a better demonstration outta the flattened list
- we make possible strings to numbers, '-1' will be -1 and ...
- we generate the postfix notation outta the numbers and operators
- evaluate the result
But all this pain is done easily thorough Calc
type in the library
from postfixcalc import Calc
calc = Calc('(-1) ^ 2')
print(calc.answer)
This is easy but Calc
type provide other cached_propertied which are just the results of the upper functions
from postfixcalc import Calc
c = Calc("2 * -1")
print(c.parsed)
print(c.extracted)
print(c.flattened)
print(c.strparenthesized)
print(c.listparenthesized)
print(c.numerized)
print(c.postfix)
print(c.answer)
print(c.stranswer)
# <ast.BinOp object at 0x7fcd313ecbe0>
# [([2], <ast.Mult object at 0x7fcd32002a70>, [(<ast.USub object at 0x7fcd32003010>, [1])])]
# ([2], <ast.Mult object at 0x7fcd32002a70>, (<ast.USub object at 0x7fcd32003010>, [1]))
# 2 * (-1)
# [2, '*', '(', '-1', ')']
# [2, '*', '(', -1, ')']
# [2, -1, '*']
# -2
# -2
Important notes
- For safety reasons, calculations are done with a timeout which is absolutely easy to change:
c = Calc('...', timeout=10)
timeout is in seconds.
answer
property returns the actual object of the answer, whether it isint
orfloat
, BUT if you want toprint
the answer, you should consider theobj to str conversion
time, it may be quite long or short depending on that obj; because of thisCalc
implements a new propery calledstranswer
which calculates the str repr with a timeout and raises exceptions if it would take long Always usestranswer
if you want the str repr of theanswer
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