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

• user2665694: Why do you need eval() ?

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.

• ninjagecko: In Python3, this is now a function. You may need to pass in a fake locals/globals dictionary, perform the assignment to a known magic value, and do fun = myFakeLocals['magic_value']
• Matti Virkkunen: @ninjagecko: His code was using Python 2 syntax so I assumed he'd like his response in the same dialect. It's trivial to "port" to Python 3 though, just add parenthesis. No need to pass in locals/globals either.
• ninjagecko: oops, indeed you are quite correct; I was confusing it with an alternative method to locals()[functionName], if you didn't know the name fun ahead of time
• ShadowRanger: eval is not solely for expressions; it's just that str arguments must be expressions, but it can also take code objects of arbitrary complexity. If you mix it with compile, you can eval arbitrary code, e.g. eval(compile(mystr, '<string>', 'exec')) works fine (and works identically on Python 2 and Python 3, unlike exec).
• Dmytro: "But before you do that, think about whether you really need dynamic code or not. By far most things can be done without.", Yes, otherwise I would be writing VBScript. "By far most things can be done without.", and would be better done with by being more parallelizable(if C had built in function-to-shellcode precompiler, threads would be much more practical to write without resorting to a higher language or making a global function for every thread proc) and not cluttering global scope with stuff only used once.

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, e.g.:

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).

• Jonno_FTW: Doing this in python 3.7 returns None and not the function object.
• ShadowRanger: @Jonno_FTW: It's not supposed to return it (def is not an expression, it doesn't produce anything assignable), it just adds it to the current scope (local if within a function, global otherwise, and you can optionally pass alternate dicts for globals or locals). eval is only expected to produce a useful return value when used with strings or with compile in 'eval' mode; in 'single' or 'exec' mode, it's not evaluating expressions.

Answer 4

If your logic is very simple (i.e., 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.