Python - exec(expression, globals=None, locals=None)

Question

I understand the basic use of eval as shown as an example in the Python standard library:

x = 1
print(eval('x+1'))
2

Could someone please provide a more concise explanation with examples for the utilisation of both the globals and locals arguements.

Answer 1

If you specify global, local namespace, they are used for global, local variables instead of current scope.

>>> x = 1
>>> d = {'x': 9}
>>> exec('x += 1; print(x)', d, d) # x => 9 (not 1)
10

NOTE: x outside the dictionary is not affected.

>>> x
1
>>> d['x']
10

Answer 2

globals and locals allow you to define the scope in which eval should operate, ie which variables should be available to it when attempting to evaluate the expression. For example:

>>> eval("x * 2", {'x': 5, 'y': 6}, {'x': 4})
8

Note that with x in local and global scope, the local version is used.