python2 to 3 use of list()

Question

I'm converting python2.7 scripts to python3. 2to3 makes these kinds of suggestions:

 result = result.split(',')
 syslog_trace("Result : {0}".format(result), False, DEBUG)
- data.append(map(float, result))
+ data.append(list(map(float, result)))
 if (len(data) > samples):
 data.pop(0)
 syslog_trace("Data : {0}".format(data), False, DEBUG)
 # report sample average
 if (startTime % reportTime < sampleTime):
- somma = map(sum, zip(*data))
+ somma = list(map(sum, list(zip(*data))))
 # not all entries should be float
 # 0.37, 0.18, 0.17, 4, 143, 32147, 3, 4, 93, 0, 0
 averages = [format(sm / len(data), '.3f') for sm in somma]

I'm sure the makers of Python3 did not want to do it like that. At least, it gives me a "you must be kidding" feeling.

Is there a more pythonic way of doing this?

• zondo: List comprehension, maybe. [float(x) for x in result]
• Mausy5043: @zondo : thanks. I'll investigate list comprehension. Does that also work for the map(sum, zip(*data))?
• zondo: Yes. Use [sum(d) for d in zip(*data)]
• jonrsharpe: That's the "quick fix" mentioned in the docs, which also suggests using list comps instead.
• Kos: 2to3 is intended to translate Python 2 code into equivalent Python 3 code. The next step is to make it well-written Python 3, but an automatic tool won't do it for you!

Answer 1

What's wrong with the unfixed somma?

2to3 cannot know how somma is going to be used, in that case, as a generator in the next line to compute averages it is OK and optimal, no need to convert it as a list.

That's the genius of python 3 list to generator changes: most people used those lists as generators already, wasting precious memory materializing lists they did not need.

 # report sample average
 if (startTime % reportTime < sampleTime):
 somma = map(sum, zip(*data))
 # not all entries should be float
 # 0.37, 0.18, 0.17, 4, 143, 32147, 3, 4, 93, 0, 0
 averages = [format(sm / len(data), '.3f') for sm in somma]

Of course the first statement, unconverted, will fail since we append a generator whereas we need a list. In that case, the error is quickly fixed.

If left like this: data.append(map(float, result)), the next trace shows something fishy: 'Data : [<map object at 0x00000000043DB6A0>]', that you can quickly fix by cnverting to list as 2to3 suggested.

2to3 does its best to create running Python 3 code, but it does not replace manual rework or produce optimal code. When you are in a hurry you can apply it, but always check the diffs vs the old code like the OP did.

The -3 option of latest Python 2 versions print warnings when an error would be raised using Python 3. It's another approach, better when you have more time to perform your migration.

Answer 2

I'm sure the makers of Python3 did not want to do it like that

Well, the makers of Python generally don't like seeing Python 2 being used, I've seen that sentiment being expressed in pretty much every recent PyCon.

Is there a more pythonic way of doing this?

That really depends on your interpretation of Pythonic, list comps seem more intuitive in your case, you want to construct a list so there's no need to create an iterator with map or zip and then feed it to list().

Now, why 2to3 chose list() wrapping instead of comps, I do not know; probably easiest to actually implement.