Read python function from a text file and assign it to variable

Question

I have a text file that contains Python function like this:

a.txt

def func():
    var = 5
    return var

And then I read this file in a Python script:

b.py

python_file = open("a.txt").read()

Now I want to assign the a.txt file's function to a variable without worrying about the function name and execute it. I tried something like this:

python_file = open("a.txt").read()
b = exec(python_file)
b()

But it didn't work, I tried execfile as well.

Answer 1

After you've exec uted the string, you can call func directly, as it has been added to your current namespace:

>>> exec("""def func():
    var = 5  # note that the semicolons are redundant and unpythonic
    return var""")
>>> func()
5

Per its documentation exec doesn't actually return anything, so there's no point assigning eg foo = exec(...) .

---

To see what names are locally defined in the code being executed, pass an empty dictionary to exec as the locals parameter:

>>> ns = {}
>>> exec("""def func():
    var = 5
    return var""", globals(), ns)
>>> ns
{'func': <function func at 0x0315F540>}

You can then assign the function and call it as you normally would:

>>> b, = ns.values()  # this will only work if only one name was defined
>>> b()
5

Answer 2

Before offering my solution, I highly warn against do this unless you know for sure there is no malicious code in a.txt .

My solution uses the execfile function to load the text file and return the first object (could be a variable or function):

def load_function(filename):
    """ Assume that filename contains only 1 function """
    global_var = dict()
    execfile(filename, global_var)
    del global_var['__builtins__']
    return next(global_var.itervalues())

# Use it
myfunction = load_function('a.txt')
print myfunction()

Update

To be a little more careful, modify the return line like the following so that it skips variables (it cannot skip class declaration, however).

    return next(f for f in global_var.itervalues() if callable(f))

Update 2

Thank you johnsharpe for pointing out that there is no execfile in Python 3. Here is a modified solution which use exec instead. This time, the function should be found in the "local" scope.

def load_function(filename):
    """ Assume that filename contains only 1 function """
    with open(filename) as f:
        file_contents = f.read()
        global_var = dict()
        local_var = dict()
        exec file_contents in global_var, local_var
        return next(f for f in local_var.itervalues() if callable(f))

# Use it
myfunction = load_function('a.txt')
print myfunction()