In python is there a way to check if a function is a "generator function" before calling it?

Question

Lets say I have two functions:

def foo():   return 'foo'  def bar():   yield 'bar' 

The first one is a normal function, and the second is a generator function. Now I want to write something like this:

def run(func):   if is_generator_function(func):      gen = func()      gen.next()      #... run the generator ...   else:      func() 

What will a straightforward implementation of is_generator_function() look like? Using the types package I can test if gen is a generator, but I wish to do so before invoking func().

Now consider the following case:

def goo():   if False:      yield   else:      return 

An invocation of goo() will return a generator. I presume that the python parser knows that the goo() function has a yield statement, and I wonder if it possible to get that information easily.

Thanks!

Answer 1

>>> import inspect >>>  >>> def foo(): ...   return 'foo' ...  >>> def bar(): ...   yield 'bar' ...  >>> print inspect.isgeneratorfunction(foo) False >>> print inspect.isgeneratorfunction(bar) True 

• New in Python version 2.6

Answer 2

>>> def foo(): ...   return 'foo' ...  >>> def bar(): ...   yield 'bar' ...  >>> import dis >>> dis.dis(foo)   2           0 LOAD_CONST               1 ('foo')               3 RETURN_VALUE         >>> dis.dis(bar)   2           0 LOAD_CONST               1 ('bar')               3 YIELD_VALUE                        4 POP_TOP                            5 LOAD_CONST               0 (None)               8 RETURN_VALUE         >>>  

As you see, the key difference is that the bytecode for bar will contain at least one YIELD_VALUE opcode. I recommend using the dis module (redirecting its output to a StringIO instance and checking its getvalue, of course) because this provides you a measure of robustness over bytecode changes -- the exact numeric values of the opcodes will change, but the disassembled symbolic value will stay pretty stable;-).