What's the difference between eval, exec, and compile?

Question

I\'ve been looking at dynamic evaluation of Python code, and come across the eval() and compile() functions, and the exec statement.

Answer 1

1. exec is not an expression: a statement in Python 2.x, and a function in Python 3.x. It compiles and immediately evaluates a statement or set of statement contained in a string. Example:

    exec('print(5)')           # prints 5.
    # exec 'print 5'     if you use Python 2.x, nor the exec neither the print is a function there
    exec('print(5)\nprint(6)')  # prints 5{newline}6.
    exec('if True: print(6)')  # prints 6.
    exec('5')                 # does nothing and returns nothing.
   

2. eval is a built-in function (not a statement), which evaluates an expression and returns the value that expression produces. Example:

    x = eval('5')              # x <- 5
    x = eval('%d + 6' % x)     # x <- 11
    x = eval('abs(%d)' % -100) # x <- 100
    x = eval('x = 5')          # INVALID; assignment is not an expression.
    x = eval('if 1: x = 4')    # INVALID; if is a statement, not an expression.
   

3. compile is a lower level version of exec and eval. It does not execute or evaluate your statements or expressions, but returns a code object that can do it. The modes are as follows:

4. compile(string, '', 'eval') returns the code object that would have been executed had you done eval(string). Note that you cannot use statements in this mode; only a (single) expression is valid.

5. compile(string, '', 'exec') returns the code object that would have been executed had you done exec(string). You can use any number of statements here.

6. compile(string, '', 'single') is like the exec mode but expects exactly one expression/statement, eg compile('a=1 if 1 else 3', 'myf', mode='single')