Moving from Python 2 to Python 3: A 4 page "cheat sheet"

  • Thread starter Mark Summerfield
  • Start date
M

Mark Summerfield

I've produced a 4 page document that provides a very concise summary
of Python 2<->3 differences plus the most commonly used new Python 3
features. It is aimed at existing Python 2 programmers who want to
start writing Python 3 programs and want to use Python 3 idioms rather
than those from Python 2 where the idioms differ.

It uses Python 3.1 syntax since that looks like being the standard for
a few years in view of the language moratorium.

The document is U.S. Letter size but will also print fine on A4
printers.

It is available as a free PDF download (no registration or anything)
from InformIT's website. Here's the direct link:
http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/promotions/python/python2python3.pdf

And of course, if you want more on Python 3, there's always the
documentation---or my book:)
"Programming in Python 3 (Second Edition)" ISBN-10: 0321680561.
 
D

Daniel Fetchinson

I've produced a 4 page document that provides a very concise summary
of Python 2<->3 differences plus the most commonly used new Python 3
features. It is aimed at existing Python 2 programmers who want to
start writing Python 3 programs and want to use Python 3 idioms rather
than those from Python 2 where the idioms differ.

It uses Python 3.1 syntax since that looks like being the standard for
a few years in view of the language moratorium.
Click to expand...

This really looks very useful, thanks a lot!
I've been wishing something like this existed for a while, really handy.

Cheers,
Daniel
 
M

Mark Dickinson

I've produced a 4 page document that provides a very concise summary
of Python 2<->3 differences plus the most commonly used new Python 3
features.
Click to expand...

Very nice indeed!

My only quibble is with the statement on the first page that
the 'String % operator is deprecated'. I'm not sure that's
true, for all values of 'deprecated'. There don't appear
to be any definite plans for getting rid of it just yet.

Mark
 
L

Lie Ryan

I've produced a 4 page document that provides a very concise summary
of Python 2<->3 differences plus the most commonly used new Python 3
features. It is aimed at existing Python 2 programmers who want to
start writing Python 3 programs and want to use Python 3 idioms rather
than those from Python 2 where the idioms differ.

It uses Python 3.1 syntax since that looks like being the standard for
a few years in view of the language moratorium.

The document is U.S. Letter size but will also print fine on A4
printers.

It is available as a free PDF download (no registration or anything)
from InformIT's website. Here's the direct link:
http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/promotions/python/python2python3.pdf

And of course, if you want more on Python 3, there's always the
documentation---or my book:)
"Programming in Python 3 (Second Edition)" ISBN-10: 0321680561.
Click to expand...

Nice.

I suggest changing the lambda example a bit, the current example says:
Python 2 Python 3
lambda (a,b): a + b lambda t: t[0] + t[1]
lambda a, b: a + b

into something like:

Python 2 Python 3
lambda (a,b),c: a + b + c lambda t, c: t[0] + t[1] + c
lambda a, b, c: a + b + c

it is unclear at first sight that it refers to tuple argument unpacking.
There should also some mention that tuple argument unpacking for regular
function (def) is also gone.



Also, I'm not sure what this change is referring to:
Python 2 Python 3
L = list(seq) L = sorted(seq)
L.sort()

L.sort is still available in python, and sorted() have been available
since python 2. Both list.sort() and sorted() are for different purpose,
and neither will be deprecated. What's the change here?
 
T

Terry Reedy

Mark said:
I've produced a 4 page document that provides a very concise summary
of Python 2<->3 differences plus the most commonly used new Python 3
features. It is aimed at existing Python 2 programmers who want to
start writing Python 3 programs and want to use Python 3 idioms rather
than those from Python 2 where the idioms differ.

It uses Python 3.1 syntax since that looks like being the standard for
a few years in view of the language moratorium.

The document is U.S. Letter size but will also print fine on A4
printers.

It is available as a free PDF download (no registration or anything)
from InformIT's website. Here's the direct link:
http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/promotions/python/python2python3.pdf

And of course, if you want more on Python 3, there's always the
documentation---or my book:)
"Programming in Python 3 (Second Edition)" ISBN-10: 0321680561.
Click to expand...

What might be even *more* helpful, with contributions from others
perhaps, would be an indication of which changes are handled
automatically by 2to3.py and which must be done by hand.

tjr
 
J

John Bokma

Mark Summerfield said:
It is available as a free PDF download (no registration or anything)
from InformIT's website. Here's the direct link:
http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/promotions/python/python2python3.pdf
Thanks!

And of course, if you want more on Python 3, there's always the
documentation---or my book:)
"Programming in Python 3 (Second Edition)" ISBN-10: 0321680561.
Click to expand...

Meh, second edition already? Haven't read the entire first edition
yet. Which is IMO a good book (I also gave it to my brother as a
present).

Only negative point (to me) so far is that in the beginning (p8-9) the
book mentions placing Python programs in C:\py3eg which gives me the
unpleasant feeling that someone is coding on Windows XP with
Administrator rights...

Anyway, will add the 2nd edition to my wish list and donate the current
one to the library in Xalapa (USBI) if they want it :)

John
 
M

Mark Summerfield

Very nice indeed!

My only quibble is with the statement on the first page that
the 'String % operator is deprecated'.  I'm not sure that's
true, for all values of 'deprecated'.  There don't appear
to be any definite plans for getting rid of it just yet.

Mark
Click to expand...

I didn't make this up:)

According to http://docs.python.org/dev/3.0/whatsnew/3.0.html
"The plan is to eventually make this the only API for string
formatting, and to start deprecating the % operator in Python 3.1."
 
M

Mark Summerfield

I've produced a 4 page document that provides a very concise summary
of Python 2<->3 differences plus the most commonly used new Python 3
features. It is aimed at existing Python 2 programmers who want to
start writing Python 3 programs and want to use Python 3 idioms rather
than those from Python 2 where the idioms differ.
Click to expand...
It uses Python 3.1 syntax since that looks like being the standard for
a few years in view of the language moratorium.
Click to expand...
The document is U.S. Letter size but will also print fine on A4
printers.
Click to expand...
It is available as a free PDF download (no registration or anything)
from InformIT's website. Here's the direct link:
http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/promotions/...
Click to expand...
And of course, if you want more on Python 3, there's always the
documentation---or my book:)
"Programming in Python 3 (Second Edition)" ISBN-10: 0321680561.
Nice.
Click to expand...

Thanks!

I suggest changing the lambda example a bit, the current example says:
Python 2                      Python 3
lambda (a,b): a + b           lambda t: t[0] + t[1]
                               lambda a, b: a + b

into something like:

Python 2                      Python 3
lambda (a,b),c: a + b + c     lambda t, c: t[0] + t[1] + c
                               lambda a, b, c: a + b + c

it is unclear at first sight that it refers to tuple argument unpacking.
Click to expand...

Your proposed example is clearer in some respects, but mine is more
minimal. And I think that anyone who _thinks_ about mine will get the
point. (The document is short, but I never claimed it was a quick
read;-)
There should also some mention that tuple argument unpacking for regular
function (def) is also gone.
Click to expand...

I probably should have, but it is hard to fit any more in... esp.
since I don't want to take anything out.
Also, I'm not sure what this change is referring to:
Python 2                 Python 3
L = list(seq)            L = sorted(seq)
L.sort()

L.sort is still available in python, and sorted() have been available
since python 2. Both list.sort() and sorted() are for different purpose,
and neither will be deprecated. What's the change here?
Click to expand...

The document is about idioms as well as changes. In this case both
approaches work in both versions, but it seems that there are still a
lot of people who don't know about sorted(), so I put it in to show it
as an idiom.
 
M

Mark Summerfield

What might be even *more* helpful, with contributions from others
perhaps, would be an indication of which changes are handled
automatically by 2to3.py and which must be done by hand.

tjr
Click to expand...

No, that's exactly what I did not want to cover and the document says
so up front. It is aimed at people who want Python 3 to come from
their own brains and fingers!

Also, the kind of info you're talking about is covered elsewhere, for
example:
http://diveintopython3.org/porting-code-to-python-3-with-2to3.html
 
M

Mark Summerfield

Meh, second edition already? Haven't read the entire first edition
yet. Which is IMO a good book (I also gave it to my brother as a
present).
Click to expand...

If it is any consolation, the second edition should have a much longer
life, now that we have the language moratorium. (I _really_ wanted to
cover 3.1.)
Only negative point (to me) so far is that in the beginning (p8-9) the
book mentions placing Python programs in C:\py3eg which gives me the
unpleasant feeling that someone is coding on Windows XP with
Administrator rights...
Click to expand...

OK, you got me there, I only use Windows for testing purposes and my
personal logon account does have Administrator rights, which I assumed
was standard for personal machines? Also, the path is short. It is
only a suggestion, it really doesn't matter where you unpack the
examples.
 
M

Mark Dickinson

I didn't make this up:)
Click to expand...

No, I didn't imagine for a second that you had!
According to http://docs.python.org/dev/3.0/whatsnew/3.0.html
"The plan is to eventually make this the only API for string
formatting, and to start deprecating the % operator in Python 3.1."
Click to expand...

I think that's a doc bug. "The plan is to ..." should read: "The plan
was
originally to ...". (Well, at least in the 3.1 version of the
"what's new in 3.0" documentation; the 3.0 version that you linked to
isn't even autogenerated any more, AFAIK, so fixes to the ReST source
for that file never make it to the web site.)

I'm a little confused myself about what's actually happening with
% formatting, but here's a fairly recent python-dev posting from
the BDFL:

http://mail.python.org/pipermail/python-dev/2009-September/092399.html

Mark
 
W

Wolodja Wentland

The document is about idioms as well as changes. In this case both
approaches work in both versions, but it seems that there are still a
lot of people who don't know about sorted(), so I put it in to show it
as an idiom.
Click to expand...

It would be quite nice if you could mark all the Python 3 idioms that
work in Python 2.X as well. This would allow readers that are still using
Python 2.X and are used to the 'old way' to adapt their coding style
accordingly. You could just add a little (2.X) after the idiom for
example.

And thanks for the nice cheat sheet! :-D

--
.''`. Wolodja Wentland <[email protected]>
: :' :
`. `'` 4096R/CAF14EFC
`- 081C B7CD FF04 2BA9 94EA 36B2 8B7F 7D30 CAF1 4EFC

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M

Mark Summerfield

I've produced a 4 page document that provides a very concise summary
of Python 2<->3 differences plus the most commonly used new Python 3
features. It is aimed at existing Python 2 programmers who want to
start writing Python 3 programs and want to use Python 3 idioms rather
than those from Python 2 where the idioms differ.

It uses Python 3.1 syntax since that looks like being the standard for
a few years in view of the language moratorium.

The document is U.S. Letter size but will also print fine on A4
printers.

It is available as a free PDF download (no registration or anything)
from InformIT's website. Here's the direct link:http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/promotions/...

And of course, if you want more on Python 3, there's always the
documentation---or my book:)
"Programming in Python 3 (Second Edition)" ISBN-10: 0321680561.
Click to expand...


I only just found out that I was supposed to give a different URL:
http://www.informit.com/promotions/promotion.aspx?promo=137519
This leads to a web page where you can download the document (just by
clicking the "Download Now" button), but if you _choose_ you can also
enter your name and email to win some sort of prize.
 
M

Mark Summerfield

No, I didn't imagine for a second that you had!


I think that's a doc bug.  "The plan is to ..." should read: "The plan
was
originally to ...".  (Well, at least in the 3.1 version of the
"what's new in 3.0" documentation;  the 3.0 version that you linked to
isn't even autogenerated any more, AFAIK, so fixes to the ReST source
for that file never make it to the web site.)

I'm a little confused myself about what's actually happening with
% formatting, but here's a fairly recent python-dev posting from
the BDFL:

http://mail.python.org/pipermail/python-dev/2009-September/092399.html
Click to expand...

Well it seems clear to me that the BDFL wants to kill of % formatting,
but wasn't able to for Python 3... So I still think it is reasonable
(1) to describe it as deprecated and (2) to only teach and use
str.format().
 
M

Mark Summerfield

It would be quite nice if you could mark all the Python 3 idioms that
work in Python 2.X as well. This would allow readers that are still using
Python 2.X and are used to the 'old way' to adapt their coding style
accordingly. You could just add a little (2.X) after the idiom for
example.
Click to expand...

Yes it would be nice, but it isn't quite so simple. To take sorted()
as just one example, it was introduced in 2.4 so arguably using it
isn't valid/idiomatic for Python 2.x programs where you care about
backwards compatibility for the Python 2.x series... But my main
reason for not wanting to do this is that the document is aimed at
people who want to write Python 3, not to encourage people to stick
with 2:)
 
M

Mark Summerfield

MarkSummerfieldwrote:



<cut>
Very handy! Am I wrong in assuming that you forgot to include that
file() is gone in favour of open()?
Click to expand...

No you are not wrong in assuming that I forgot that:-(

My lame excuse is that file() was introduced for isinstance() testing
and similar, and never really as a replacement for open().

Anyway, I have now added:

fh = file(fname, mode) | fh = open(fname, mode)

I've sent a new PDF with this change to InformIT, so hopefully it'll
become available soon from
http://www.informit.com/promotions/promotion.aspx?promo=13751
 
M

Mark Summerfield

No you are not wrong in assuming that I forgot that:-(

My lame excuse is that file() was introduced for isinstance() testing
and similar, and never really as a replacement for open().

Anyway, I have now added:

    fh = file(fname, mode) | fh = open(fname, mode)

I've sent a new PDF with this change to InformIT, so hopefully it'll
become available soon fromhttp://www.informit.com/promotions/promotion.aspx?promo=13751
Click to expand...

Oops wrong URL again, should have been:
http://www.informit.com/promotions/promotion.aspx?promo=137519
.... time to go offline and sleep ...
 
J

John Posner

Mark, I add my thanks to those of the other responders. If you find space,
you might consider adding another str.format() feature:

Goal: place integer 456 flush-right in a field of width 8

Py2: "%%%dd" % 8 % 456
Py3: "{0:{1}d}".format(456, 8)

With str.format(), you don't need to nest one formatting operation within
another. A little less mind-bending, and every little bit helps!

-John
 

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