Cchem: 1 revision

2019-09-19T18:47:01Z

1 revision

Speleo3: str.translate

2019-08-28T07:08:13Z

str.translate

Cchem: 1 revision

2019-03-04T03:58:18Z

1 revision

Speleo3: created

2019-02-19T09:02:48Z

created

New page

This page is about porting your existing scripts and plugins to Python 3, while keeping Python 2 compatibility.
The strategies described here have also been used to port most of the scripts in [https://github.com/Pymol-Scripts/Pymol-script-repo Pymol-script-repo].

Recipe overview:

# Make a backup of your scripts (of course)
# 2to3 -w ...
# fix imports
# fix str/bytes mixing
# ...

= 2to3 =

[https://docs.python.org/3/library/2to3.html 2to3] is an automated conversion tool and part of every Python installation. It will for example replace <code>print "Hello"</code> with <code>print("Hello")</code>. We'll exclude all <code>import</code> fixes here, because we want to use a Python 2 compatible fix instead.

Run in a terminal:

2to3 -w --nofix=imports --nofix=urllib --nofix=future myscript.py

= Imports =

Place (the relevant subset of) this block at the beginning of each of your script files:

<syntaxhighlight lang="python" style="font-size: 70%">
from __future__ import division
from __future__ import print_function
from __future__ import absolute_import

import sys
if sys.version_info[0] < 3:
import urllib2
from urllib2 import URLError, HTTPError
from urllib import quote
from idlelib.TreeWidget import TreeItem, TreeNode
from Tkinter import *
import Tkinter
import tkSimpleDialog
import tkFileDialog
import tkMessageBox
import tkColorChooser
import Queue
import ttk
from StringIO import StringIO
else:
import urllib.request as urllib2
from urllib.error import URLError, HTTPError
from urllib.parse import quote
from idlelib.tree import TreeItem, TreeNode
from tkinter import *
import tkinter as Tkinter
from tkinter import simpledialog as tkSimpleDialog
from tkinter import filedialog as tkFileDialog
import tkinter.messagebox as tkMessageBox
import tkinter.colorchooser as tkColorChooser
import queue as Queue
import tkinter.ttk as ttk
from io import StringIO
</syntaxhighlight>

'''Some important notes:'''

* <code>from __future__ import division</code> will change the behavior of integer division, <code>3/2 == 1</code> becomes <code>3/2 == 1.5</code>. Use <code>//</code> if you need [https://python-reference.readthedocs.io/en/latest/docs/operators/floor_division.html floor division] (<code>3//2 == 1</code> in both Python 2 and 3).
* <code>io.StringIO</code> is not an exact replacement for <code>StringIO.StringIO</code>, it doesn't accept [https://docs.python.org/3/library/stdtypes.html#bytes bytes] input. You might need <code>io.BytesIO</code> instead.

= str vs. bytes =

This can be the most tricky part. In Python 3, all strings are unicode, and <code>unicode</code> was renamed to <code>str</code> and <code>str</code> to <code>bytes</code>. Also, Python 2 allowed mixing of <code>unicode</code> and <code>bytes</code> and automatically encoded/decoded between them when needed. Python 3 doesn't do that, you have to encode and decode yourself, and pay attention to binary vs. text when writing files.

{| class="wikitable"
!
! Python 2
! Python 3
! Literals (2 and 3 compatible)
|-
! Unicode type
| unicode
| str
| u"..."
|-
! Binary type
| str (alias bytes)
| bytes
| b"..."
|}

Code examples that works in Python 2 and 3:

<syntaxhighlight lang="python">
text = u"Greek letter alpha: \u03B1"
binary = u"Greek letter alpha: \u03B1".encode("utf-8")
binary = b"Greek letter alpha: \xce\xb1"
text = b"Greek letter alpha: \xce\xb1".decode("utf-8")
</syntaxhighlight>

Use binary mode when reading or writing bytes from/to files:

<syntaxhighlight lang="python">
# text mode
with open("file.txt", "w") as f:
f.write(u"Hello World\n")
f.write(b"Hello World\n".decode("utf-8"))

# binary mode
with open("file.bin", "wb") as f:
f.write(b"Hello World\n")
f.write(u"Hello World\n".encode("utf-8"))
</syntaxhighlight>

The [https://docs.python.org/3/library/subprocess.html subprocess] modules opens streams in binary mode, unless you pass <code>universal_newlines=True</code>:

<syntaxhighlight lang="python">
# text mode
p = subprocess.Popen(['echo', 'Hello World'],
universal_newlines=True,
stdout=subprocess.PIPE)
text = p.stdout.read()

# binary mode
p = subprocess.Popen(['echo', 'Hello World'],
stdout=subprocess.PIPE)
binary = p.stdout.read()
</syntaxhighlight>

= string module =

Most [https://docs.python.org/2/library/string.html#deprecated-string-functions <code>string</code> module functions are gone], use <code>str</code> methods instead.

Examples:

{| class="wikitable"
! Python 2 only
! Python 2 and 3
|-
| <code>string.strip(" Hello ")</code>
| <code>" Hello ".strip()</code>
|-
| <code>string.split("A B C")</code>
| <code>"A B C".split()</code>
|-
| <code>string.join(["A", "B", "C"], " ")</code>
| <code>" ".join(["A", "B", "C"])</code>
|}

Note: PyMOL has an interim solution for this, which will be removed in a future version:

<syntaxhighlight lang="python" style="font-size: 70%">
if sys.version_info[0] == 3:
import string
for attr in ['capitalize', 'count', 'find', 'index', 'lower',
'replace', 'rstrip', 'split', 'strip', 'upper', 'zfill']:
setattr(string, attr, getattr(str, attr))
string.letters = string.ascii_letters
string.lowercase = string.ascii_lowercase
string.join = lambda words, sep=' ': sep.join(words)
string.atoi = int
</syntaxhighlight>

= Hex formatting of floats =

This works in Python 2, but is an error in Python 3:

<syntaxhighlight lang="python">
>>> '%x' % 12.34
TypeError: %x format: an integer is required, not float
</syntaxhighlight>

Solution: Convert floats to <code>int</code>:

<syntaxhighlight lang="python">
>>> '%x' % int(12.34)
</syntaxhighlight>

← Older revision	Revision as of 18:47, 19 September 2019
(No difference)

← Older revision		Revision as of 07:08, 28 August 2019
Line 170:		Line 170:
	>>> '%x' % int(12.34)		>>> '%x' % int(12.34)
	</syntaxhighlight>		</syntaxhighlight>
		+
		+	= str.translate =
		+
		+	The ''deletechars'' argument of [https://docs.python.org/2/library/stdtypes.html#str.translate str.translate] is gone.
		+
		+	{\| class="wikitable"
		+	! Python 2 only
		+	! Python 2 and 3
		+	\|-
		+	\| <code>"Hello".translate(None, "aeiou")</code>
		+	\| <code>"".join(c for c in "Hello" if c not in "aeiou")</code>
		+	\|-
		+	\|}

← Older revision	Revision as of 03:58, 4 March 2019
(No difference)

2to3 - Revision history

Cchem: 1 revision

Speleo3: str.translate

Cchem: 1 revision

Speleo3: created