How should sp.if be used?

Question

I find I run into some unexpected behavior when using sp.if. e.g.

        a = False
        sp.if 1 > 2:
            a = True

seems to result in a == True.

The example below is doing so in the Store Value contract template.

# Store Value - Example for illustrative purposes only.

import smartpy as sp

class StoreValue(sp.Contract):
    def __init__(self, value):
        self.init(storedValue = value)

    @sp.entry_point
    def replace(self, params):
        
        a = False
        sp.if 1 > 2:
            a = True
        
        sp.if a:
            self.data.storedValue = params.value

    @sp.entry_point
    def double(self, params):
        self.data.storedValue *= 2

    @sp.entry_point
    def divide(self, params):
        sp.verify(params.divisor > 5)
        self.data.storedValue /= params.divisor

if "templates" not in __name__:
    @sp.add_test(name = "StoreValue")
    def test():
        c1 = StoreValue(12)
        scenario = sp.test_scenario()
        scenario.h1("Store Value")
        scenario += c1
        scenario += c1.replace(value = 15)
        scenario.p("Some computation").show(c1.data.storedValue * 12)
        scenario += c1.replace(value = 25)
        scenario += c1.double()
        scenario += c1.divide(divisor = 2).run(valid = False)
        scenario.verify(c1.data.storedValue == 50)
        scenario += c1.divide(divisor = 6)
        scenario.verify(c1.data.storedValue == 8)
    ```

Answer 1

The short answer is that you need to use local variables.

a = sp.local('a', False)
sp.if ...:
    a.value = True
...

Otherwise, a is computed too soon by Python and not by SmartPy / Michelson.