E731 do not assign a lambda expression, use a def

The recommendation in PEP-8 you are running into is:

Always use a def statement instead of an assignment statement that binds a lambda expression directly to a name.

Yes:

def f(x): return 2*x 

No:

f = lambda x: 2*x 

The first form means that the name of the resulting function object is specifically 'f' instead of the generic '<lambda>'. This is more useful for tracebacks and string representations in general. The use of the assignment statement eliminates the sole benefit a lambda expression can offer over an explicit def statement (i.e. that it can be embedded inside a larger expression)

Assigning lambdas to names basically just duplicates the functionality of def - and in general, it's best to do something a single way to avoid confusion and increase clarity.

The legitimate use case for lambda is where you want to use a function without assigning it, e.g:

sorted(players, key=lambda player: player.rank)

In general, the main argument against doing this is that def statements will result in more lines of code. My main response to that would be: yes, and that is fine. Unless you are code golfing, minimising the number of lines isn't something you should be doing: go for clear over short.

Here is the story, I had a simple lambda function which I was using twice.

a = map(lambda x : x + offset, simple_list)
b = map(lambda x : x + offset, another_simple_list)

This is just for the representation, I have faced couple of different versions of this.

Now, to keep things DRY, I start to reuse this common lambda.

f = lambda x : x + offset
a = map(f, simple_list)
b = map(f, another_simple_list)

At this point my code quality checker complains about lambda being a named function so I convert it into a function.

def f(x):
    return x + offset
a = map(f, simple_list)
b = map(f, another_simple_list)

Now the checker complains that a function has to be bounded by one blank line before and after.

def f(x):
    return x + offset

a = map(f, simple_list)
b = map(f, another_simple_list)

Here we have now 6 lines of code instead of original 2 lines with no increase in readability and no increase in being pythonic. At this point the code checker complains about the function not having docstrings.

In my opinion this rule better be avoided and broken when it makes sense, use your judgement.

Lattyware is absolutely right: Basically PEP-8 wants you to avoid things like

f = lambda x: 2 * x

and instead use

def f(x):
    return 2 * x

However, as addressed in a recent bugreport (Aug 2014), statements such as the following are now compliant:

a.f = lambda x: 2 * x
a["f"] = lambda x: 2 * x

Since my PEP-8 checker doesn't implement this correctly yet, I turned off E731 for the time being.