how to define a function from a string using python

Question

this is my code :

a = \
'''def fun():\n
    print 'bbb'
'''
eval(a)

fun()

but it shows error :

Traceback (most recent call last):
  File "c.py", line 8, in <module>
    eval(a)
  File "<string>", line 1
    def fun():
      ^
SyntaxError: invalid syntax

so what can i do ,

thanks

Answer 1

eval() with a string argument is only for expressions. If you want to execute statements, use exec :

exec """def fun():
  print 'bbb'
"""

But before you do that, think about whether you really need dynamic code or not. By far most things can be done without.

Answer 2

Eval evalutes only expressions, while exec executes statements.

So you try something like this

a = \
'''def fun():\n
    print 'bbb'
'''
exec a

fun()

Answer 3

Non-expression eval arguments must be compile -ed first; a str is only processed as an expression, so full statements and arbitrary code require compile .

If you mix it with compile , you can eval arbitrary code, eg:

eval(compile('''def fun():
    print 'bbb'
''', '<string>', 'exec'))

The above works fine and works identically on Python 2 and Python 3, unlike exec (which is a keyword in Py2, and a function in Py3).

Answer 4

If your logic is very simple (ie, one line), you could eval a lambda expression:

a = eval("lambda x: print('hello {0}'.format(x))")
a("world") # prints "hello world"

As others have mentioned, it is probably best to avoid eval if you can.