How to run printed code Python

Question

I am trying to create a function that runs printed code. For example,

def test(mylist):
    mydict = {}
    for i in range(mylist):
        mydict[i] = ('x'+'%s' %i)
        print ("%s = %s" %(mydict[i], i))
test(3)

Output:

x0 = 0
x1 = 1
x2 = 2

I basically want my function to run these printed commands and set x0=0, x1=1, and so on. I could not think of a way to do it. Thanks!

Edit:

I edited the code based on your dictionary idea. But, it does not still seem to be helping to solve for the gradient of a given function. Can you please help me on this?

import sympy as sy
def test(mylist):
    d = {}
    for i in range(mylist):
        d['x'+str(i)] = 'sy.symbols(x%s, real=True)' %i
    return d
test(3)

f = (1/3*x0**6 - 2.1*x0**4 + 4*x0**2 + 
            x0*x1 - 4*x1**2 + 4*x1**4 + x2)
gf = [sy.diff(f, x0), sy.diff(f, x1), sy.diff(f, x2)]
gf

Trying to create dynamic variables is a _baaaaad_ idea. Why not just use the dictionary you're creating? — Christian Dean, Feb 10 '18 at 02:33
[Dupe?](https://stackoverflow.com/questions/9383740/what-does-pythons-eval-do) — user202729, Feb 10 '18 at 02:34
Well, in this case, the dictionary makes sense. But, I want to set `x0` to a long function, where dictionary will be useless. — tcokyasar, Feb 10 '18 at 02:35
Eh, not really @user202729. The user's not explicitly asking about `eval`, although that is the function is would need to `use` (but definitely not _should!_). There are duplicates out there though, just keep searcin' :-) — Christian Dean, Feb 10 '18 at 02:36
@user202729 Huh? I'm really sure what your asking here. Can you be more specific. Or better yet, edit your answer to show exactly what you want. — Christian Dean, Feb 10 '18 at 02:38
Then how do you intend to use those `x0, x1, x2` variables? By typing out their names? In that case you can as well type the definition out explicitly. — user202729, Feb 10 '18 at 02:42
The same thing could be done in your newly posted example, @user8028576 . just assign the return value of each call to `sy.symbols` to a key in a dictionary. I'm sorry, but i'm still no sure why you can't use a dictionary? — Christian Dean, Feb 10 '18 at 02:43
I still think a dictionary is not doing what I need. Am I really missing it? @ChristianDean — tcokyasar, Feb 10 '18 at 02:59
Hold up, @user8028576. Gimme five and I'll post an answer to clear things up :-) — Christian Dean, Feb 10 '18 at 03:04
Alright, @user8028576. Sorry about the wait, my answers up though. — Christian Dean, Feb 10 '18 at 03:17

score 0 · Answer 1 · answered Feb 10 '18 at 02:38

0

As pointed out in comments, dictionaries are the way to go. Stylistically, get in the habit of using return rather than printing results.

I've made some changes to your code, which should illustrate these points:

def test(mylist):
    d = {}
    for i in range(mylist):
        d['x'+str(i)] = i
    return d

test(3)  # {'x0': 0, 'x1': 1, 'x2': 2}

answered Feb 10 '18 at 02:38

jpp

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Your code could be condensed if you used a dictionary comprehension `return {'x' + str(i) for i in range(mylist)}`. – Christian Dean Feb 10 '18 at 02:40
Agreed - but one step at a time! – jpp Feb 10 '18 at 02:40
Yeah, I agree, but we don't know _for sure_ if the OP is new to Python ;-) – Christian Dean Feb 10 '18 at 02:41

score 0 · Answer 2 · answered Feb 10 '18 at 03:16

0

Trying to create dynamic variables is almost always a bad idea. It's dangers and often unnecessarily pollutes a namespace. However, I agree that just using plain variables here would be easier. Thus, you can return a list of values from test, and unpack them into three variables (x0, x1, and x2):

Create a list of your values:

def test(mylist):
    return [sy.symbols('x' + str(n), real=True) for n in range(mylist)]

And then create your gf list using the values in the list:

x0, x1, x2 = test(3)
f = (1/3*x0**6 - 2.1*x0**4 + 4*x0**2 + x0*x1 - 4*x1**2 + 4*x1**4 + x2)
gf = [sy.diff(f, x0), sy.diff(f, x1), sy.diff(f, x2)]

answered Feb 10 '18 at 03:16

Christian Dean

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Thanks for your solution, but, writing: `x0, x1, x2 = test(3)` is what I want to avoid. I could also do the same. What if I have 100 variables? – tcokyasar Feb 10 '18 at 03:22
@user8028576 You _could_ use a list comprehension to create your `gf` list (`gf = [sy.diff(f, v) for v in test(3)]`), but I'm not sure how you'd evaluate your `f` equation? Is there a way it can be generalized? – Christian Dean Feb 10 '18 at 03:24
Arranging gf is the secondary task. If I could generalize xi= 'sy.symbols(xi, real=True). Then, it could be meaningful. – tcokyasar Feb 10 '18 at 04:13

How to run printed code Python

2 Answers2