Python Eval() Function

Python Eval() Function

The eval() function in python evaluates any python statement. The 1^st argument to eval function must be a string , bytes or code object.

Example 1:   Python Code below

>>> variable = ‘5+(6/3)’

>>> eval(variable)

7.0 à Result

If you directly pass a statement to eval() function it won’t execute and throws below error.

>>> eval(5+(6/3))

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

TypeError: eval() arg 1 must be a string, bytes or code object

If you quote the input statement, then it works.

>>> eval('5+(6/3)')

7.0 à Result

The complete Syntax of eval() function is given below. Where parameter 2^nd & 3^rd are optional.

eval(expression, globals, locals)

expression: A string, bytes or code object.

globals: A dictionary referring global methods & Variable.

locals: A dictionary referring local methods & Variable.

When we use eval() function without 2^nd & 3^rd parameter, the application is executed in current scope and the available methods and variable to the program can be checked using below code:

>>> eval(‘dir()’)

Output : ['__annotations__', '__builtins__', '__doc__', '__loader__', '__name__', '__package__', '__spec__']

To see the available methods & variable in globals & locals dictionary run below commands:

>>> eval('globals()')

>>> eval('locals()')

Example 2: I am creating my own function and a variable here. Now globals() & locals() have my function and variable in it as it is running in current context.  Since output is dictionary, it is embraced by curly braces.

>>> def myfunction():

...     print('This is my function')

...

>>> myvar = 123

>>> globals()

{'__name__': '__main__', '__doc__': None, '__package__': None,'__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, 'myfunction': <function myfunction at 0x7f3659ddab00>, 'myvar': 123}

>>> locals()

{'__name__': '__main__', '__doc__': None, '__package__': None,'__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, 'myfunction': <function myfunction at 0x7f3659ddab00>, 'myvar': 123}

You can also restrict the method available to statement in eval() first parameters by explicitly mentioning them.

>>> from math import *

>>> eval('dir()') àReturns all from dir and math.

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

>>> eval('dir()',{'sin':sin})   à Here in explicitly defining to have Sin method available only.

 ['__builtins__', 'sin']

Example 3: In Below code I am import math library but I in my eval() I am only making Sin function to be available to expression.So it throwing error .

>>> from math import *

>>> a=5

>>> eval('sqrt(a)')

2.23606797749979

>>> eval('sqrt(a)',{'sin':sin})

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "<string>", line 1, in <module>

NameError: name 'sqrt' is not defined