What's the main difference between py2 vs py3?

neuralbrand · 8 Jan 2018, 20:56

From a beginner point of view what is the main changes in our Drawbot code when using Python 2 and Python 3?

Tx

frederik · 8 Jan 2018, 22:00

For beginners there is not such a big difference:

Strings

All strings are unicode string, so no converting back and forth, but this is actually the best thing!!

print

print('hello world') requires brackets ( and )

divisions

Divisions are always floats so no more rounding, this is also a good thing: 1/3-> 0.333333 instead of 0

You keep the old behaviour with 1//3

If you dive deeper:

dictionaries

dict.keys(), dict.values(), dict.items() are not a lists anymore

Looping (a for loop or a while loop) over a dictionary and changing content of that dictionary is tricky:

for key in list(myDict.keys()):
    myDict[key] = "changed" + myDict[key]

There is of course more

neuralbrand · 9 Jan 2018, 12:13

Thanks! I'll look more into it.

thom · 11 Jan 2018, 16:18

I noticed today iterate a dict works also different.
While translating scripts from py2 to py3 this was a thing...

If the keys are integers in py2 they will be sorted.
In py3 the will stay in order as appended to dict.

print {1:"A",0:"B"} 
> {0: 'B', 1: 'A'}

vs

print({1:"A",0:"B"})
> {1: 'A', 0: 'B'}

(also it seems print() is not coloured in snippet)

justvanrossum · 11 Jan 2018, 22:17

@thom It's actually not generally true that keys get sorted in Py2. The order of a dict is in implementation detail that you should not rely on. If you need sorted keys: get sorted keys!

d = {1:"A",0:"B"}
print(d)
> {1: 'A', 0: 'B'}
print(d.keys())
> dict_keys([1, 0])
print(sorted(d))
> [0, 1]

(Indeed print() not getting colorized may be a bug in the syntax colorizer. Or it's not yet fully Py 3 compatible.)

frederik · 12 Jan 2018, 11:16

@justvanrossum improved... and requested here https://github.com/isagalaev/highlight.js/issues/1468