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Ride the carousel of folly with a Python migration

mortoray.com

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Re: Ride the carousel of folly with a Python migration

#3

Post by bigmanwalter »

Python 2 > 3 is the next Y2K. My colleague and I have started offering Python 2 > 3 consulting services as some of our clients weren't prepared to make the migration themselves. With Django announcing that they will drop Python 2.7 support, this might become one of our more popular services!

Re: Ride the carousel of folly with a Python migration

#5

Post by deathanatos »

> The only somewhat problematic code was this:

  for line in iter( handle.stdout.readline, '' ):
    self.output.put( ( handle, line ) )
> This no longer terminated an emitted lots of odd values (one number repeatedly). It turns out it returns an empty b'' now when empty, as opposed to a ''. This could have been avoided had this API sensibly returned None instead of an empty string.

For any observers, this is much more idiomatically written:

  for line in handle.stdout:
    self.output.put((handle, line))
That is, the default iteration over a file-like object is line by line. Frankly, I wonder how many Python programmers are aware that iter() has a two-arg form; this is the first time I've ever seen it used (and I had to look it up myself).

> [PyPI]

His solution of maintaining separate code bases is … unusual. I feel like a cursory glance around the library landscape shows that most of the projects out there are maintaining a single codebase that supports 2.7+. six greatly simplifies most of this. I'm not aware of any projects that are supporting Python 2 and 3 concurrently by separating out their code completely.

> I presumed that I’d just open a “Migration from Python 2 to 3 Guide” and follow the instructions. Alas, there is no such guide. There appears to be no central resource whatsoever on the porting of code from version 2 to version 3.

An I'm Feeling Lucky search on that exact quote pulls up the migration guide in Python's official documentation.[1]

[1]: https://docs.python.org/3/howto/pyporting.html

Re: Ride the carousel of folly with a Python migration

#6

Post by agf »

deathanatos wrote: post #5

> The only somewhat problematic code was this: for line in iter( handle.stdout.readline, '' ): self.output.put( ( handle, line ) ) > This no longer terminated an emitted lots of odd values (one number repeatedly). It turns out it returns an empty b'' now when empty, as opposed to a ''. This could have been avoided had this API sensibly returned None instead of an empty string. For any observers, this is much more idi…

In addition to what you've pointed out, he claims that 2to3 didn't do anything to his code, and yet he had to fix print statements. 2to3 definitely fixes print statements; he likely didn't read the usage or docs at all. By default it prints a diff, you have to use 2to3 -w if you want it to modify files on disk -- I certainly don't want tools modifying files in-place by default, so that seems like the right API to me.

Re: Ride the carousel of folly with a Python migration

#8

Post by pdonis »

This statement in the article...

"there is nothing in the PyPI package description that actually indicates what version of Python is being targeted"

is incorrect. There are Trove classifiers[1] for both versions, and indeed for every minor version number of both versions. There are even Python 2 and 3 classifiers that end in "Only", which look like they would exactly fit the author's use case.

[1] https://pypi.python.org/pypi?%3Aaction=list_classifiers

Re: Ride the carousel of folly with a Python migration

#9

Post by lowmagnet »

I ran a 2to3 last week on a codebase I've been working for the past six months. Took me one try to get that I was missing a flag. 'git diff' cured me of thinking it wrote anything, and I found '-w' shortly after.

I guess a masochist could do something like pipe 1 to 'patch -p0' or something…

I ran into the same unicode issues as the author, and dumped all the code that handled it because I didn't need it in 3.

I also reduced the whole codebase by some 150 lines because of not needing to do a bunch of extra work I originally designed in when working through the code. This was more due to fresh eyes.

ETA: the code was on 2.7 because a library I needed wasn't yet available under python 3 (FuelSDK from SalesForce) until November, and I completely missed this. 1.0 version supports py 3, but it wasn't in pip.

Re: Ride the carousel of folly with a Python migration

#10

Post by freyir »

pdw wrote: post #2

He claims there's no migration guide, but when I google on the phrase he used, "Migration from Python 2 to 3 Guide", or any variation of that phrase, the first hit is https://docs.python.org/3/howto/pyporting.html I wonder why he didn't find it.

And when 2to3 didn't work, he didn't attempt `2to3 --help`?

I guess if everything went smoothly, he wouldn't have had a blog post to write.

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