Thread: python var directly to evaluation

is possible have python take math symbol entered variable , put directly operation, ex

x = input('enter operator')
y = input('enter 1st #: ')
z = input('enter second #: ')

want print out result. can do
y _ z

can manually make bunch of 'if' statements , fill in appropriate operation in underlined blank.. however, there lots of operations check for. there equivalent doing

print y x z

there python plugs in , sees expression, , evaluates it?

the easiest way, eval, in fact, you're allowing use expressions in input because of function "input"

python 2.6
"raw_input" : reads input string
"input" : reads input python expression

(will deprecated in favor of)
python 3.x
"input" : reads input string.

see yourself:

php code:

evalme = input("what you want to do?")

print evalme 


code:

what want do? 2+3   5

this method unsafe, because having directly interpreted input (either eval or 2.6 "input") may lead malicious input "import os; <delete file>"

i can manually make bunch of 'if' statements , fill in appropriate operation in underlined blank.. however, there lots of operations check for. there equivalent doing

that's because you're thinking on procedural, switch gears functional, , can do:

python 2.6

php code:

#!/usr/bin/env python

# dictionary of tasks
tasks = {
    "+" : lambda x, y: x + y,
    "-" : lambda x, y: x - y,
    "*" : lambda x, y: x * y,
    "/" : lambda x, y: x / y
}

operator = raw_input("enter an operator >").strip()
leftmost = int(raw_input("enter the left-most oparand >"))
rightmost = int(raw_input("enter the right-most oparand >"))

# call the function on the dictionary
# with both arguments
result = tasks[operator](leftmost, rightmost)

# print result
print "%d %s %d = %d" % (leftmost, operator, rightmost, result) 


the reason why works, because dictionary, works getting key (the operator in case) , returning it's value (a function in case). therefor, calling:

php code:

f = tasks["+"] 


will yield function addition, , executing in other function:

php code:

print f(2, 3) # prints 5 


all done in single line, anonymous functions.

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