Eval/Exec with assigning variable - Python

Question

The below piece of code is aimed at converting a factorial into its products. E.g. "4!" --> "(4*3*2*1)". This code does not work due to the line exec(codeToRun). However, if I instead put the value of codeToRun in place of exec(codeToRun) then it works perfectly so why doesn't exec work?

Doesn't work ↓

def checkSpecialChars(char, stringToCheck, codeToRun):
    while char in stringToCheck:
        currentString=""
        for i in range(len(stringToCheck)):
            if stringToCheck[i]==char:
                try:
                    eval(codeToRun)
                except:
                    exec(codeToRun)
                    print(stringToCheck)
                currentString=""
                break
            if stringToCheck[i].isdigit():
                currentString+=stringToCheck[i]
            else:
                currentString=""
    return stringToCheck

Does work ↓

def checkSpecialChars(char, stringToCheck, codeToRun):
    while char in stringToCheck:
        currentString=""
        for i in range(len(stringToCheck)):
            if stringToCheck[i]==char:
                try:
                    eval(codeToRun)
                except:
                    stringToCheck = stringToCheck[:i-len(currentString)] + "(" + "*".join(str(integer) for integer in range(int(currentString),0,-1)) + ")" + stringToCheck[i+1:]
                print(stringToCheck)
                currentString=""
                break
            if stringToCheck[i].isdigit():
                currentString+=stringToCheck[i]
            else:
                currentString=""
    return stringToCheck

EDIT #1 The number of factorials can be more than one and the number of digits in each factorial can be more than one as well.

Input: "11!/10!"

Expected Output: "(11*10*9*8*7*6*5*4*3*2*1)/(10*9*8*7*6*5*4*3*2*1)"

EDIT #2 I have added a print statement outputting the string as seen in the two pieces of code. Now when I run the program and enter 4!, the program pauses (as if it was an infinite loop). I then press CTRL+C to exit the program and it decides to output 4!. This then happens every time I press CTRL+C so the line must be running because the print statement occurs but it remains at 4!.

Is '4!' the only test case you have? Is there only ever a single factorial per string with the format of digit(s) and then a single '!' character? — G_M
– G_M, Commented Jun 22, 2018 at 21:13
No. The number of digits can be anything (e.g. 51) and the number of factorials can also be anything. E.g. "51!/50!" is a possibility. — Dan
– Dan, Commented Jun 22, 2018 at 21:14
Can you show what the expected output would be in that case then? Or even a case with two (or more) smaller factorials? — G_M
– G_M, Commented Jun 22, 2018 at 21:16
I've edited the post to include an expected output in that case. — Dan
– Dan, Commented Jun 22, 2018 at 21:19

Paul Panzer · Accepted Answer · 2018-06-23 17:14:47Z

3

Let's have a quick look at the docs:

Help on built-in function exec in module builtins:

exec(source, globals=None, locals=None, /) Execute the given source in the context of globals and locals.
The source may be a string representing one or more Python statements
or a code object as returned by compile().
The globals must be a dictionary and locals can be any mapping,
defaulting to the current globals and locals.
If only globals is given, locals defaults to it.

and

Help on built-in function locals in module builtins:

locals() Return a dictionary containing the current scope's local variables.
NOTE: Whether or not updates to this dictionary will affect name lookups in
the local scope and vice-versa is *implementation dependent* and not
covered by any backwards compatibility guarantees.

This final note would appear to explain your troubles.

To be more specific, you are calling exec with one parameter, so it will execute in the default environment of locals() on top of globals(). It is important to realize, that these are not necessarily identical with the actual global and local scopes, but may in theory be proxies or copies or whatever. Now, if you, for example, assign a variable one of these dictionaries gets updated accordingly. What the note in the docs for locals says, is that there is no guarantee that this update will be propagated to the actual local scope. And this I think is why your program doesn't work.

How to work around this:

Based on the above one easy fix is

(1) make a contract with yourself that codeToRun assigns to stringToCheck.

(2) keep a reference to the instance of locals() you pass to exec

(3) use this to explicitly set stringToCheck

So your except block would look somewhat like

    l = locals()
    exec(codeToRun, globals(), l)
    stringToCheck = l['stringToCheck']

edited Jun 23, 2018 at 17:14

answered Jun 22, 2018 at 21:40

Paul Panzer

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5 Comments

Dan Over a year ago

I don't quite understand this. Could you please explain what this means further?

Dan Over a year ago

Yes that makes more sense, thank you. So is there a way of doing this (with or without using eval/exec) in a way that keeps the function non-specific to factorials (because the function itself is intended to find the leading number before a character and do an action based on that) ?

Dan Over a year ago

Thank you for the attempt but unfortunately it does the exact same thing as just using exec(codeToRun).

Paul Panzer Over a year ago

@Dan I checked and it works. Please try again and be careful to do it right. Start from your second version. Remove the body of the except block, i.e. the line starting with stringToCheck = and copy it to a string variable --- ctr, say --- outside the function. Copy and paste the three lines I've posted to the except block and make sure to get indentation right. Now test with parameters that worked before, for example '!', '11!/10!', ctr

Dan Over a year ago

Fantastic! It works perfectly, thank you. I realised I made a stupid mistake when copying your code the first time.

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