Fix python typo

[robertzk]

No description provided.

[AmplabJenkins]

Can one of the admins verify this patch?

[srowen]

Clearly this has to change. I don't know much Python so would be interested to hear the answer myself: is it certain that acc.add(x) evaluates to true so that f(x) is always evaluated?

[laserson]

It looks to me like acc.add(x) will always evaluate to None [1], so the function will never evaluate f(x). A hacky way to fix this may be to change the lambda to

lambda x: not acc.add(x) and f(x)

because not None evaluates to True, but this certainly won't make things more clear for the reader. The best fix would be do define a separate function

def g(x):
    acc.add(x)
    return f(x)

data.map(g)

[srowen]

@robertzk what do you think of that fix? + @ilganeli just because it looks like you created the example, although this bit may have been inadvertent copy and paste.

[robertzk]

I like the separate function; there really is no good way to write multi-line lambdas in Python.

[robertzk]

@srowen What do you think of this?

[srowen]

Certainly looks like something that stands a change of working, and approved by @laserson. I'll wait a little for any other comments.

[vanzin]

Looks fine, but if you want a crazy one-liner:

data.map(lambda x: (acc.add(x), f(x))[1])

Don't do that at home, though. We already have Perl for that kind of shenanigans.

[davies]

Nice catch, LGTM, thanks!

[srowen]

Oh I realized the variable name is accum in this example and needs to be fixed from acc on usage. I can fix on merge.

[davies]

@srowen Good catch, I saw that, then forget it immediately, really need to take a rest..

[srowen]

Heh, actually the Scala and Java examples are wrong too. They should be respectively:

val accum = sc.accumulator(0)
data.map { x => accum += x; f(x) }

Accumulator<Integer> accum = sc.accumulator(0);
data.map(x -> { accum.add(x); return f(x); });

... and I can port the spelling fix to their comments too.