Python exec() built-in function

The exec() function in Python is a built-in function that allows you to dynamically execute Python code stored in a string. This can be very powerful for scenarios where you want to execute code provided by users, generate and run code at runtime, or even build mini interpreters or scripting environments within your application. However, it should be used with caution as it can potentially introduce security risks if not handled properly.

From the Python 3 documentation

This function supports dynamic execution of Python code. object must be either a string or a code object. If it is a string, the string is parsed as a suite of Python statements which is then executed (unless a syntax error occurs) [...].


exec(object[, globals[, locals]])
  • object: The string containing the Python code to be executed.
  • globals (optional): A dictionary representing the global namespace. If not provided, it uses the current global namespace.
  • locals (optional): A dictionary representing the local namespace. If not provided, it uses the current local namespace.

Basic Usage

code_to_execute = "print('Hello, exec()!')"
# Output: Hello, exec()!

Executing Multiple Statements

code = """
for i in range(5):
# Output: 0
#         1
#         2
#         3
#         4

Modifying Variables

x = 10
code = "x += 5"
print(x)  # Output: 15

Using globals and locals

x = 5
code = "x = x * 2"
globals_dict = {"x": 10}
locals_dict = {"x": 20}
exec(code, globals_dict, locals_dict)
print(x)             # Output: 5 (unchanged)
print(globals_dict)  # Output: {'x': 10}
print(locals_dict)   # Output: {'x': 40}

Dynamic Function Creation

def create_dynamic_function(name, args):
    code = f"def {name}({', '.join(args)}): return sum({args})"

create_dynamic_function("add_numbers", ["a", "b", "c"])
result = add_numbers(2, 3, 5)
print(result)  # Output: 10

Error Handling

code = """
except NameError as e:
    print(f"Error: {e}")
# Output: Error: name 'undefined_variable' is not defined

Security Note

user_input = input("Enter code to execute: ")
exec(user_input)  # Caution: This can be a security risk if not properly sanitized.

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