How to send a value to exec's running code

Question

First, I have two classes like this:

class A(object):
    def foo(self, context):
        print(context)

class B(object):
    def foo(self, context):
        print(context)

Notice the context variable here.

Second, I want to call A and B in exec like this:

body = """
a = A()
a.foo()
b = B()
b.foo()
"""
exec(body)

I also have some other classes which use context.

Which means that I don't want the context variable appeared in the body code. It's been generated in other place. And I want to send it by exec or other methods.

So how can I do this?

Since the parameter `context` is not optional, `a.foo()` will always fail. — blhsing, Oct 18 '19 at 04:02
Yes, so how can I send the context to foo by exec, I can modify the foo method, this is not importent. — wsf1990, Oct 18 '19 at 04:04
The code which is generating your body code will need to add atleast some placeholder in line `a.foo()` so that you can add value to that when calling exec — MohitC, Oct 18 '19 at 04:11
Yes, or I can change the foo method, I can control this A class. — wsf1990, Oct 18 '19 at 04:19

blhsing · Accepted Answer · 2019-10-18T04:35:18.617

1

You can make context a global variable instead:

class A(object):
    def foo(self):
        print(context)

class B(object):
    def foo(self):
        print(context)

body = """
a = A()
a.foo()
b = B()
b.foo()
"""
context = 'foobar'
exec(body)

This outputs:

foobar
foobar

edited Oct 18 '19 at 04:35

answered Oct 18 '19 at 04:12

blhsing

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Thanks,Your answer maybe right,and I'll test it soon.But I have many places will use this `context` variable,Not just in A,So dose it have some other methods ? – wsf1990 Oct 18 '19 at 04:16
I don't add context in foo method's param, Because I think can the context be defined in other place? – wsf1990 Oct 18 '19 at 04:32
I had test this, Which can't work, Told me that context is not defined, Because A and B not at the same place as exec. – wsf1990 Oct 18 '19 at 04:33
Does your test code not look like this working demo? https://repl.it/@blhsing/ExcitableMessyNewsaggregator – blhsing Oct 18 '19 at 04:34
More like this:https://repl.it/@shoufengwei/TechnicalBigApplicationsoftware – wsf1990 Oct 18 '19 at 04:38
So the classes are defined in a different module. In that case you have to import the classes from that module and assign your external value to `context` as an attribute of the module object: https://repl.it/@blhsing/TechnicalBigApplicationsoftware – blhsing Oct 18 '19 at 04:42
@blhsing why `object` in `class A(object)` ? . Any study link... – PIG Oct 18 '19 at 04:44
1

@PIG `object` is the base class for all new-style classes. You can read about new vs. old-style classes here: https://stackoverflow.com/questions/54867/what-is-the-difference-between-old-style-and-new-style-classes-in-python – blhsing Oct 18 '19 at 04:46
if i am not using `object` inside class still it is giving same result. – PIG Oct 18 '19 at 04:49
@PIG For the purpose of this question it makes no difference. New-style classes have several features that are not available to old-style classes. [This answer](https://stackoverflow.com/a/19950198/6890912) summarizes them well. – blhsing Oct 18 '19 at 04:53

How to send a value to exec's running code

1 Answers1