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exec() in Python
Exec function can dynamically execute code of python programs. The code can be passed in as string or object code to this function. The object code is executed as is while the string is first parsed and checked for any syntax error. If no syntax error, then the parsed string is executed as a python statement.
Syntax for exec() Function
exec(object, globals, locals)
Where
Object − A string or a code object passed onto the method.
globals − A dictionary of available global methods and variables.
locals − A dictionary of available local methods and variables.
Passing String
In the below example we pass a single line of code as string to the exec() function. Itr gets parsed and executed to give the output.
x = 9
exec ('print(5*x)')Output
Running the above code gives us the following result −
45
Passing Code Object
Now we see how to pass a block of code with multiple code statements. As it is a code object, it gets executed directly giving the result. Please note how we have used \n and space to create a python code block with proper indention.
Example
prog_block = 'x = 3 \nif(x < 5): \n print x*x' exec(prog_block)
Output
Running the above code gives us the following result −
9
Without Global and Local Parameters
When we do not pass any value for global and local parameter we get the default available functions as per the package imported into the program. In the below example we see the code giving us all the available functions when both global and local parameter values are skipped.
from time import *
exec("print(dir())")Running the above code gives us the following result −
['In', 'Out', '_', '__', '___', '__builtin__', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '_dh', '_exit_code', '_i', '_i1', '_i10', '_i11', '_i12', '_i13', '_i14', '_i15', '_i16', '_i17', '_i18', '_i19', '_i2', '_i20', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'clock', 'copysign', 'cos', 'cosh', 'ctime', 'daylight', 'degrees', 'e', 'erf', 'erfc', 'exit', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'get_ipython', 'gmtime', 'hypot', 'isinf', 'isnan', 'ldexp', 'lgamma', 'localtime', 'log', 'log10', 'log1p', 'mktime', 'modf', 'pi', 'pow', 'prog', 'prog_block', 'quit', 'radians', 'sin', 'sinh', 'sleep', 'sqrt', 'strftime', 'strptime', 'struct_time', 'tan', 'tanh', 'time', 'timezone', 'trunc', 'tzname', 'x']
Applying Restrictions with Global Parameters
We can restrict the access to any of the functions of the imported module by passing on an empty dictionary as the global parameter. In this case the result will show only the built-in function and it will not show any of the function from the imported module. That is how we restrict and make the function more secure.
Example
from time import *
exec("print(dir())",{})Output
Running the above code gives us the following result −
['__builtins__']
Allowing only Selected Functions
Next we see how we can apply only certain chosen functions to the exec() method from the imported module. In the below example we allow only the required function as a parameter with the Global option. The localtime() function is part of the time module which is imported in the program.Example
from time import *
exec("print lclt()",{"lclt":localtime})Output
Running the above code gives us the following result −
time.struct_time(tm_year=2019, tm_mon=7, tm_mday=19, tm_hour=12, tm_min=33, tm_sec=53, tm_wday=4, tm_yday=200, tm_isdst=0)
Passing local parameters
We can also restrict the usage of various functions form the imported modules by using the local parameter and excluding the built in functions completely. In the below example we choose None as the value for built-in global parameter.
Example
from time import *
exec("print(dir())", {"__builtins__" : None}, {"gtime": gmtime, "print": print, "dir": dir})Running the above code gives us the following result −
['dir', 'gtime', 'print']
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