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Python exec() Function

The exec() method executes a dynamically created program, which is either a string or a code object.

note

The exec() function accepts large blocks of code, unlike the eval() function which only accepts a single expression

Syntax​

exec(object, globals, locals)

exec() Parameters​

Python exec() function parameters:

ParameterConditionDescription
objectRequiredA String, or a code object
globalsOptionalA dictionary containing global parameters
localsOptionalA dictionary containing local parameters

exec() Return Value​

Python exec() function does not return any value.

Examples​

Example 1: executing a simple print statement​

exec('print("Hello, World!")')

Example 2: defining a variable within the exec() context​

exec('a =  5')

Example 3: accessing a variable defined outside the exec() context​

a =  10
exec('print(a)')  # Output:  10

Example 4: using globals and locals to control the execution context of exec()​

global_vars = {"x":  10, "y":  20}
local_vars = {"x":  5}
exec("print(x)", global_vars, local_vars)  # Output:  5, because locals takes precedence over globals

Example 5: exec() with a multi-line input program​

We can pass a multi-line input program to the exec() method with the help of \n. But we need to use the compile() method to compile the program first.

# get a multi-line program as input
program = input('Enter a program:')

# compile the program in execution mode
b = compile(program, 'something', 'exec')

# execute the program
exec(b)

output

Enter a program:'a = 5\nb=10\nprint("Sum =", a+b)'
90

Example 6: check usable code with exec()​

It is a good thing to verify the methods and variables that can be utilized with the exec() function.

You can accomplish this by employing the dir() function, which lists all the attributes and methods of an object

For example:

# import all the methods from math library
from math import *

# check the usable methods  
exec('print(dir())')

output

['__annotations__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'comb', 'copysign', 'cos', 'cosh', 'degrees', 'dist', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'gcd', 'hypot', 'inf', 'isclose', 'isfinite', 'isinf', 'isnan', 'isqrt', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'log2', 'modf', 'nan', 'perm', 'pi', 'pow', 'prod', 'radians', 'remainder', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'tau', 'trunc']

Example 7: block not necessary methods and variable in exec()​

Most of the time, we don't need all the methods and variables with exec().

We can block these unnecessary methods and variables by passing the optional globals and locals parameter to the exec() method.

For more information, check locals() function and globals() function.

# import methods from the math library
from math import *

# use cbrt() method 
mycode='''a = cbrt(9)
print(a)'''

# empty dictionary (global parameter) to restrict the cbrt() method
exec(mycode, {})

output

Traceback (most recent call last):
  File "main.py", line 9, in <module>
    exec(mycode, {})
  File "<string>", line 1, in <module>
NameError: name 'cbrt' is not defined

Observe that in this example we have restricted the use of the cbrt() method in exec() although we have imported the whole math library.

Example 8: using necessary methods and variables in exec()​

You can also make the necessary methods and variables available to use with the exec() method.

To do this, we need to pass the locals dictionary to exec().

from math import *

# set globals parameter to none
globalsParameter = {'__builtins__' : None}

# set locals parameter to take only print() and dir()
localsParameter = {'print': print, 'dir': dir}

# print the accessible method directory
exec('print(dir())', globalsParameter, localsParameter)

output

['dir', 'print']

Notice that we have blocked all the global builtin methods with the code:

globalsParameter = {'__builtins__' : None}

but, we have allowed two methods print() and dir() to be executable with the code:

localsParameter = {'print': print, 'dir': dir}

Finally, we have passed the dir() method inside print() method and then passed it to the exec() method to print a list of usable methods.

The globalsParameter and localsParameter here are optional parameters that we have used with the exec() method to print only the methods that we want to access.

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