Eval not working on multi-line string

问题:

I am having issues with executing a multi-line string with the python eval function/

code = ''' 

def main():
  print "this is a test"

main()

'''

eval(code)

Traceback (most recent call last):
  File "<pyshell#12>", line 1, in <module>
    eval(code)
  File "<string>", line 3
    def main():
      ^
SyntaxError: invalid syntax

回答1:

eval can only evaluate Python expressions, not statements. A function definition is a statement, not an expression.

Use exec to execute Python statements.

See the Top-level components document, which differentiates (among others) between file input and expression input:

file_input ::=  (NEWLINE | statement)*

This syntax is used in the following situations:

[...]

• when parsing a string passed to the exec statement;

and

[...] The string argument to eval() must have the following form:

eval_input ::=  expression_list NEWLINE*

Do NOT use this to execute untrusted user-supplied text. eval() and exec are not guarded against malicious users, and they can and will take over the web process if you use this.

In fact, there is no 'safe' way to ever do this, other than running the code in a throw-away virtual machine with all services firmly bolted shut. Run a new virtual machine for new code, throw away the whole VM when done or after a timeout.