Question

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What’s the difference between eval, exec, and compile in Python?

I known that

1. eval is a function
2. exec is a statement

And the simple usage of both is :

eval('1+2')
exec 'print 1+2'

But there are other usages that I can’t understand.

1. Using a variable to store a function name, and using this variable to
   call the function
   eg:

   def test():
       print 'hello world'
   func = 'test'
   func = eval(func)
   func() # this will call test()
   

   I type(func) after
   func = eval(func)
   it returns
   <type 'function'>
   I read the document of eval, but I don’t known why the eval can do
   this.

2. Using a variable to store a module name, and using this variable to import the module.
   eg.

   m = 'sys'
   exec "import " +　m
   

   Is this the reason:
   import module_name is a statement, not expression?
   and:
   eval does only to calculate a expression
   exec does to run the statement in the str?

Answer 1

The part of your question about storing the function name can be explained by the fact that this would be equivalent:

def test():
    print 'hello world'
func = test
func() # this will call test()

The call to eval() in your example is no different than a call like:

y = eval('x + 1')

I believe your second question is the same as this one, and the answers might be helpful.