How to compile a string of Python code into a module whose functions can be called?

Question

In Python, I have a string of some Python source code containing functions like:

mySrc = '''
def foo():
    print("foo")

def bar():
    print("bar")
'''

I'd like to compile this string into some kind of module-like object so I can call the functions contained in the code.

Here's pseudo-code for what I'd like to do:

myMod = myCompile(mySrc)
myMod.foo()

Is this possible in Python? I've tried this, but it does not work:

myMod = compile(mySrc, '', 'exec')
myMod.foo()

This produces an error message like this:

<code object <module> at 0x104154730, file "", line 1>Traceback (most recent call last):
  File "myfile.py", line XX, in run
    myMod.foo()
AttributeError: 'code' object has no attribute 'foo'

thefourtheye · Accepted Answer · 2013-11-08 01:53:43Z

28

You have to both compile and execute it:

myMod = compile(mySrc, '', 'exec')
exec(myMod)
foo()

You can pass dicts to exec to stop foo from “leaking” out. Combine it with a module object created using types.ModuleType:

from types import ModuleType
…
compiled = compile(mySrc, '', 'exec')
module = ModuleType("testmodule")
exec(compiled, module.__dict__)

edited Nov 8, 2013 at 1:53

thefourtheye

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answered Nov 8, 2013 at 1:49

Ry-♦

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1 Comment

Todd Ditchendorf Over a year ago

The second option listed here is especially helpful, because: If the source code string contains import statements, the second option listed here executes them and allows the rest of the source code in the string to use symbols/objects thus imported. AFAICT, that is not the case in the first option. (I should have mentioned this requirement in my initial question, but your excellent/detailed answer here provided the info I needed anyway. Perfect.)

Ken Kinder · Accepted Answer · 2013-11-08 01:53:51Z

1

In Python 2, you want the magical compiler package:

>>> import compiler
>>> mod = compiler.parseFile("doublelib.py")
>>> mod
Module('This is an example module.\n\nThis is the docstring.\n',
       Stmt([Function(None, 'double', ['x'], [], 0,
                      'Return twice the argument',
                      Stmt([Return(Mul((Name('x'), Const(2))))]))]))
>>> from compiler.ast import *
>>> Module('This is an example module.\n\nThis is the docstring.\n',
...    Stmt([Function(None, 'double', ['x'], [], 0,
...                   'Return twice the argument',
...                   Stmt([Return(Mul((Name('x'), Const(2))))]))]))
Module('This is an example module.\n\nThis is the docstring.\n',
       Stmt([Function(None, 'double', ['x'], [], 0,
                      'Return twice the argument',
                      Stmt([Return(Mul((Name('x'), Const(2))))]))]))
>>> mod.doc
'This is an example module.\n\nThis is the docstring.\n'
>>> for node in mod.node.nodes:
...     print node
...
Function(None, 'double', ['x'], [], 0, 'Return twice the argument',
         Stmt([Return(Mul((Name('x'), Const(2))))]))
>>> func = mod.node.nodes[0]
>>> func.code
Stmt([Return(Mul((Name('x'), Const(2))))])

And in Python 3, it's built right in.

answered Nov 8, 2013 at 1:53

Ken Kinder

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3 Comments

Todd Ditchendorf Over a year ago

Thanks, this is very interesting. But I have my source code in an in-memory string (not in a file on disk). In this case I will also prefer to use a very high-level solution like the answer above. Very good to learn about this, tho. Will be useful. :)

Ken Kinder Over a year ago

In Py2, it's just compiler.parse for an in-memory string, instead of parseFile. In Python 3, I believe you just use compile() and pass something like '<string>' as the filename.

doug65536 Over a year ago

@KenKinder Yeah, but then what? Pass it to exec, right? exec returns None, where's the module?

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