Python Functions

This chapter is from the book

Python Distilled

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This chapter is from the book

This chapter is from the book 

Python Distilled

Learn More Buy

5.22 Dynamic Code Execution and Creation

The exec(str [, globals [, locals]]) function executes a string containing arbitrary Python code. The code supplied to exec() is executed as if the code actually appeared in place of the exec operation. Here’s an example:

a = [3, 5, 10, 13]
exec('for i in a: print(i)')

The code given to exec() executes within the local and global namespace of the caller. However, be aware that changes to local variables have no effect. For example:

def func():
    x = 10
    exec("x = 20")
    print(x)      # Prints 10

The reasons for this have to do with the locals being a dictionary of collected local variables, not the actual local variables (see the previous section for more detail).

Optionally, exec() can accept one or two dictionary objects that serve as the global and local namespaces for the code to be executed, respectively. Here’s an example:

globs = {'x': 7,
           'y': 10,
           'birds': ['Parrot', 'Swallow', 'Albatross']
           }

locs = { }

# Execute using the above dictionaries as the global and local namespace
exec('z = 3 * x + 4 * y', globs, locs)
exec('for b in birds: print(b)', globs, locs)

If you omit one or both namespaces, the current values of the global and local namespaces are used. If you only provide a dictionary for globals, it’s used for both the globals and locals.

A common use of dynamic code execution is for creating functions and methods. For example, here’s a function that creates an __init__() method for a class given a list of names:

def make_init(*names):
    parms = ','.join(names)
    code = f'def __init__(self, {parms}):\n'
    for name in names:
        code += f' self.{name} = {name}\n'
    d = { }
    exec(code, d)
    return d['__init__']

# Example use
class Vector:
    __init__ = make_init('x','y','z')

This technique is used in various parts of the standard library. For example, namedtuple(), @dataclass, and similar features all rely on dynamic code creation with exec().