Interesting little "gotcha" with generators

[Kenneth McDonald]

I recently had need to write the following code:

def compileOuter(self):
if False: yield None
else: return

"compileOuter" is a generator function which is implemented in
various classes. In this particular class, it always yields nothing.
However, none of the following work:

def compileOuter(self):
return

def compileOuter(self):
pass

def compileOuter(self):
yield

The first two don't work because in order to define a generator, you
must have a yield statement inside it. The last doesn't work because
every "yield" must have an argument.

I've been using "return" in generators ever since I started using
generators, but on reflection, it seems to me that such a thing is in
some ways inconsistent; "return" is (conceptually, at least
originally) a function statement, where "return" by itself really
stands in for "return None". But in generators, it is being used as a
control flow command. For example, you can't have "return" with an
argument inside a generator.

Too bad "return" wasn't entirely forbidden within generators, and
"yield" without an argument mandated instead. Oh well, too let now I
suppose...


Cheers,
Ken

 

[Duncan Booth]

I recently had need to write the following code:

def compileOuter(self):
if False: yield None
else: return

"compileOuter" is a generator function which is implemented in
various classes. In this particular class, it always yields nothing.
However, none of the following work:

def compileOuter(self):
return

def compileOuter(self):
pass

def compileOuter(self):
yield

This would work:

def compileOuter(self):
return iter(())

and depending on how it is used, you might also get away with:

def compileOuter(self):
return []

A generator is simply a function which when called returns an iterator, so
you can use any other function which returns an iterator in its place.

 

[Aahz]

I recently had need to write the following code:

def compileOuter(self):
if False: yield None
else: return

What's wrong with

def foo():
if False: yield None

 

[Will McGugan]

I recently had need to write the following code:

def compileOuter(self):
if False: yield None
else: return

"compileOuter" is a generator function which is implemented in various
classes. In this particular class, it always yields nothing. However,
none of the following work:

def compileOuter(self):
return

def compileOuter(self):
pass

def compileOuter(self):
yield

The first two don't work because in order to define a generator, you
must have a yield statement inside it. The last doesn't work because
every "yield" must have an argument.

I've been using "return" in generators ever since I started using
generators, but on reflection, it seems to me that such a thing is in
some ways inconsistent; "return" is (conceptually, at least originally)
a function statement, where "return" by itself really stands in for
"return None". But in generators, it is being used as a control flow
command. For example, you can't have "return" with an argument inside a
generator.

Too bad "return" wasn't entirely forbidden within generators, and
"yield" without an argument mandated instead. Oh well, too let now I
suppose...

Would this work?

def compilerOuter(self):
raise StopIteration

Will McGugan

 

[Paul Rubin]

What's wrong with

def foo():
if False: yield None

Does the following work?

def foo():
raise StopIteration

 

[Aahz]

Does the following work?

def foo():
raise StopIteration

Nope.