Python 实用宝典

Question 1

Python has at least six ways of formatting a string:

In [1]: world = "Earth"

# method 1a
In [2]: "Hello, %s" % world
Out[2]: 'Hello, Earth'

# method 1b
In [3]: "Hello, %(planet)s" % {"planet": world}
Out[3]: 'Hello, Earth'

# method 2a
In [4]: "Hello, {0}".format(world)
Out[4]: 'Hello, Earth'

# method 2b
In [5]: "Hello, {planet}".format(planet=world)
Out[5]: 'Hello, Earth'

# method 2c
In [6]: f"Hello, {world}"
Out[6]: 'Hello, Earth'

In [7]: from string import Template

# method 3
In [8]: Template("Hello, $planet").substitute(planet=world)
Out[8]: 'Hello, Earth'

A brief history of the different methods:

printf-style formatting has been around since Pythons infancy
The Template class was introduced in Python 2.4
The format method was introduced in Python 2.6
f-strings were introduced in Python 3.6

My questions are:

Is printf-style formatting deprecated or going to be deprecated?
In the Template class, is the substitute method deprecated or going to be deprecated? (I’m not talking about safe_substitute, which as I understand it offers unique capabilities)

Similar questions and why I think they’re not duplicates:

Python string formatting: % vs. .format — treats only methods 1 and 2, and asks which one is better; my question is explicitly about deprecation in the light of the Zen of Python
String formatting options: pros and cons — treats only methods 1a and 1b in the question, 1 and 2 in the answer, and also nothing about deprecation
advanced string formatting vs template strings — mostly about methods 1 and 3, and doesn’t address deprecation
String formatting expressions (Python) — answer mentions that the original ‘%’ approach is planned to be deprecated. But what’s the difference between planned to be deprecated, pending deprecation and actual deprecation? And the printf-style method doesn’t raise even a PendingDeprecationWarning, so is this really going to be deprecated? This post is also quite old, so the information may be outdated.

This phrase was removed later, in commit Close #4966: revamp the sequence docs in order to better explain the state of modern Python. This might seem like a sign that a plan to deprecate % formatting was back on the cards… but diving into the bug tracker reveals that the intent was the opposite. On the bug tracker, the author of the commit characterises the change like this:

changed the prose that describes the relationship between printf-style formatting and the str.format method (deliberately removing the implication that the former is any real danger of disappearing – it’s simply not practical for us to seriously contemplate killing it off)

In other words, we’ve had two consecutive changes to the %-formatting docs intended to explicitly emphasise that it will not be deprecated, let alone removed. The docs remain opinionated on the relative merits of different kinds of string formatting, but they’re also clear the %-formatting isn’t going to get deprecated or removed.

What’s more, the most recent change to that paragraph, in March 2017, changed it from this…

The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). Using the newer formatted string literals or the str.format interface helps avoid these errors. These alternatives also provide more powerful, flexible and extensible approaches to formatting text.

… to this:

The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). Using the newer formatted string literals, the str.format interface, or template strings may help avoid these errors. Each of these alternatives provides their own trade-offs and benefits of simplicity, flexibility, and/or extensibility.

Notice the change from “helps avoid” to “may help avoid”, and how the clear recommendation of .format and f-strings has been replaced by fluffy, equivocal prose about how each style “provides their own trade-offs and benefits”. That is, not only is a formal deprecation no longer on the cards, but the current docs are openly acknowledging that % formatting at least has some “benefits” over the other approaches.

I’d infer from all this that the movement to deprecate or remove % formatting has not only faltered, but been defeated thoroughly and permanently.

Question 3

The new .format() method is meant to replace the old % formatting syntax. The latter has been de-emphasised, (but not officially deprecated yet). The method documentation states as much:

This method of string formatting is the new standard in Python 3, and should be preferred to the % formatting described in String Formatting Operations in new code.

(Emphasis mine).

To maintain backwards compatibility and to make transition easier, the old format has been left in place for now. From the original PEP 3101 proposal:

Backwards Compatibility

Backwards compatibility can be maintained by leaving the existing mechanisms in place. The new system does not collide with any of the method names of the existing string formatting techniques, so both systems can co-exist until it comes time to deprecate the older system.

Note the until it comes time to deprecate the older system; it hasn’t been deprecated, but the new system is to be used whenever you write new code.

The new system has as an advantage that you can combine the tuple and dictionary approach of the old % formatter:

"{greeting}, {0}".format(world, greeting='Hello')

and is extensible through the object.__format__() hook used to handle formatting of individual values.

Note that the old system had % and the Template class, where the latter allows you to create subclasses that add or alter its behaviour. The new-style system has the Formatter class to fill the same niche.

Python 3 has further stepped away from deprecation, instead giving you warning in the printf-style String Formatting section:

Note: The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). Using the newer formatted string literals or the str.format() interface helps avoid these errors. These alternatives also provide more powerful, flexible and extensible approaches to formatting text.

Python 3.6 also added formatted string literals, which in-line the expressions into the format strings. These are the fastest method of creating strings with interpolated values, and should be used instead of str.format() wherever you can use a literal.

Question 4

The % operator for string formatting is not deprecated, and is not going to be removed – despite the other answers.
Every time the subject is raised on Python development list, there is strong controversy on which is better, but no controversy on whether to remove the classic way – it will stay. Despite being denoted on PEP 3101, Python 3.1 had come and gone, and % formatting is still around.

The statements for the keeping classic style are clear: it is simple, it is fast, it is quick to do for short things. Using the .format method is not always more readable – and barely anyone – even among the core developers, can use the full syntax provided by .format without having to look at the reference Even back in 2009, one had messages like this: http://mail.python.org/pipermail/python-dev/2009-October/092529.html – the subject had barely showed up in the lists since.

2016 update

In current Python development version (which will become Python 3.6) there is a third method of string interpolation, described on PEP-0498. It defines a new quote prefix f"" (besides the current u"", b"" and r"").

Prefixing a string by f will call a method on the string object at runtime, which will automatically interpolate variables from the current scope into the string:

>>> value = 80
>>> f'The value is {value}.'
'The value is 80.'

Question 5

Guido’s latest position on this seems to be indicated here:

What’s New In Python 3.0

PEP 3101: A New Approach To String Formatting

A new system for built-in string formatting operations replaces the % string formatting operator. (However, the % operator is still supported; it will be deprecated in Python 3.1 and removed from the language at some later time.) Read PEP 3101 for the full scoop.

And the PEP3101 itself, which has the last modified dating back to (Fri, 30 Sep 2011), so no progress as of late on that one, I suppose.

Question 6

Looking at the older Python docs and PEP 3101 there was a statement that the % operator will be deprecated and removed from the language in the future. The following statement was in the Python docs for Python 3.0, 3.1, and 3.2:

Since str.format() is quite new, a lot of Python code still uses the % operator. However, because this old style of formatting will eventually be removed from the language, str.format() should generally be used.

If you go to the same section in Python 3.3 and 3.4 docs, you will see that statement has been removed. I also cannot find any other statement anywhere else in the documentation indicating that the operator will be deprecated or removed from the language. It’s also important to note that PEP3101 has not been modified in over two and a half years (Fri, 30 Sep 2011).

Update

PEP461 Adding % formatting to bytes and bytearray is accepted and should be part of Python 3.5 or 3.6. It’s another sign that the % operator is alive and kicking.

Question 7

I do most of my programming in Python 3.x on Windows 7, but now I need to use the Python Imaging Library (PIL), ImageMagick, and wxPython, all of which require Python 2.x.

Can I have both Python 2.x and Python 3.x installed in Windows 7? When I run a script, how would I “choose” which version of Python should run it? Will the aforementioned programs be able to handle multiple versions of Python installed at once? I have searched for hours and hours for how to do this to no avail.

Thanks.

Question 8

I found that the formal way to do this is as follows:

Just install two (or more, using their installers) versions of Python on Windows 7 (for me work with 3.3 and 2.7).

Follow the instuctions below, changing the parameters for your needs.

Create the following environment variable (to default on double click):

Name:  PY_PYTHON
Value: 3

To launch a script in a particular interpreter, add the following shebang (beginning of script):

#! python2

To execute a script using a specific interpreter, use the following prompt command:

> py -2 MyScript.py

To launch a specific interpreter:

> py -2

To launch the default interpreter (defined by the PY_PYTHON variable):

> py

Resources

Documentation: Using Python on Windows

PEP 397 – Python launcher for Windows

Question 9

What I did was download both 2.7.6 and 3.3.4. Python 3.3.4 has the option to add the path to it in the environment variable so that was done. So basically I just manually added Python 2.7.6.

How to…

Start > in the search type in environment select “Edit environment variables to your account”1
Scroll down to Path, select path, click edit.
Add C:\Python27; so you should have paths to both versions of Python there, but if you don’t this you can easily edit it so that you do….. C:\Python27;C:\Python33;
Navigate to the Python27 folder in C:\ and rename a copy of python.exe to python2.exe
Navigate to the Python34 folder in C:\ and rename a copy of python.exe to python3.exe
Test: open up commmand prompt and type python2 ….BOOM! Python 2.7.6. exit out.
Test: open up commmand prompt and type python3 ….BOOM! Python 3.4.3. exit out.

Note: (so as not to break pip commands in step 4 and 5, keep copy of python.exe in the same directory as the renamed file)

Question 10

I have multiple versions in windows. I just change the exe name of the version I’m not defaulting to.

python.exe –> python26.exe

pythonw.exe –> pythonw26.exe

As for package installers, most exe installers allow you to choose the python install to add the package too. For manual installation check out the –prefix option to define where the package should be installed:

http://docs.python.org/install/index.html#alternate-installation-windows-the-prefix-scheme

Question 11

If you use Anaconda Python, you can easily install various environments.

Say you had Anaconda Python 2.7 installed and you wanted a python 3.4 environment:

conda create -n py34 python=3.4 anaconda

Then to activate the environment:

activate py34

And to deactive:

deactivate py34

(With Linux, you should use source activate py34.)

Links:

Download Anaconda Python

Instructions for environments

Question 12

To install and run any version of Python in the same system follow my guide below.

For example say you want to install Python 2.x and Python 3.x on the same Windows system.

Install both of their binary releases anywhere you want.
- When prompted do not register their file extensions and
- do not add them automatically to the PATH environment variable
Running simply the command python the executable that is first met in PATH will be chosen for launch. In other words, add the Python directories manually. The one you add first will be selected when you type python. Consecutive python programs (increasing order that their directories are placed in PATH) will be chosen like so:
- py -2 for the second python
- py -3 for the third python etc..
No matter the order of “pythons” you can:
- run Python 2.x scripts using the command: py -2 (Python 3.x functionality) (ie. the first Python 2.x installation program found in your PATH will be selected)
- run Python 3.x scripts using the command: or py -3 (ie. the first Python 3.x installation program found in your PATH will be selected)

In my example I have Python 2.7.14 installed first and Python 3.5.3. This is how my PATH variable starts with:

PATH=C:\Program Files\Microsoft MPI\Bin\;C:\Python27;C:\Program Files\Python_3.6\Scripts\;C:\Program Files\Python_3.6\;C:\ProgramData\Oracle\Java\javapath;C:\Program Files (x86)\Common Files\Intel\Shared

…

Note that Python 2.7 is first and Python 3.5 second.

So running python command will launch python 2.7 (if Python 3.5 the same command would launch Python 3.5).
Running py -2 launches Python 2.7 (because it happens that the second Python is Python 3.5 which is incompatible with py -2). Running py -3 launches Python 3.5 (because it’s Python 3.x)
If you had another python later in your path you would launch like so: py -4. This may change if/when Python version 4 is released.

Now py -4 or py -5 etc. on my system outputs: Requested Python version (4) not installed or Requested Python version (5) not installed etc.

Hopefully this is clear enough.

Question 13

Starting version 3.3 Windows version has Python launcher, please take a look at section 3.4. Python Launcher for Windows

Question 14

Here’s what you can do:

Install cmder. Open and use Cmder as you would with you cmd terminal. Use the command alias to create command aliases.

I did the following:

alias python2 = c:\python27\python.exe
alias python3 = c:\python34\python.exe

And that’s it! ;-)

Question 15

I actually just thought of an interesting solution. While Windows will not allow you to easily alias programs, you can instead create renamed batch files that will call the current program.

Instead of renaming the executable which will break a lot of thing including pip, create the file python2.bat in the same directory as the python2.exe. Then add the following line:

%~dp0python %*

What does this archaic syntax mean? Well, it’s a batch script, (Windows version of bash). %~dp0 gets the current directory and %* will just pass all the arguments to python that were passed to the script.

Repeat for python3.bat

You can also do the same for pip and other utilities, just replace the word python in the file with pip or whathever the filename. The alias will be whatever the file is named.

Best of all, when added to the PATH, Windows ignores the extension so running

python3

Will launch the python3 version and and the command python2 will launch the python2 version.

BTW, this is the same technique Spyder uses to add itself to the path on Windows. :)

Question 16

You can install multiple versions of Python one machine, and during setup, you can choose to have one of them associate itself with Python file extensions. If you install modules, there will be different setup packages for different versions, or you can choose which version you want to target. Since they generally install themselves into the site-packages directory of the interpreter version, there shouldn’t be any conflicts (but I haven’t tested this). To choose which version of python, you would have to manually specify the path to the interpreter if it is not the default one. As far as I know, they would share the same PATH and PYTHONPATH variables, which may be a problem.

Note: I run Windows XP. I have no idea if any of this changes for other versions, but I don’t see any reason that it would.

Question 17

What I have done on my own windows computer where I have Python 2.7 and Python 3.4 installed is I wrote a simple .bat file in the same directory as my Python.exe files. They look something like,

cmd /k "c:\python27\python.exe" %*

The %* allows you to add arguments (Python files) afterwards. I believe /k keeps the prompt open after it finishes running the script. Then I save that as python27.bat Then I go to my Python 3 directory and make a bat file there. Now in my command line I can write

Python27 helloworld.py

Or

Python34 helloworld.py

And they will run in their respective versions of Python. Make sure that c:\python27 and c:\python34 are in your environment variables.

I got my answer from here

Question 18

I did this in three steps by following the instructions here: This is all taken directly from here: http://ipython.readthedocs.io/en/stable/install/kernel_install.html. I’m currently running Python 2.x on Windows 8 and have Anaconda 4.2.13 installed.

1) First install the latest version of python:

conda create -n python3 python=3 ipykernel

2) Next activate python3

activate python3

3) Install the kernel:

python -m ipykernel install --user

If you have Python 3 installed and want to install 2, switch the 2 and the 3 above. When you open a new notebook, you can now choose between Python 2 or 3.

Question 19

Check your system environment variables after installing Python, python 3’s directories should be first in your PATH variable, then python 2.

Whichever path variable matches first is the one Windows uses.

As always py -2 will launch python2 in this scenario.

Question 20

I have encountered that problem myself and I made my launchers in a .bat so you could choose the version you want to launch.

The only problem is your .py must be in the python folder, but anyway here is the code:

For Python2

@echo off
title Python2 Launcher by KinDa
cls
echo Type the exact version of Python you use (eg. 23, 24, 25, 26)
set/p version=
cls
echo Type the file you want to launch without .py (eg. hello world, calculator)
set/p launch=
path = %PATH%;C:\Python%version%
cd C:\Python%version%
python %launch%.py
pause

For Python3

@echo off
title Python3 Launcher by KinDa
cls
echo Type the exact version of Python you use (eg. 31, 32, 33, 34)
set/p version=
cls
echo Type the file you want to launch without .py (eg. hello world, calculator)
set/p launch=
cls
set path = %PATH%:C:\Python%version%
cd C:\Python%version%
python %launch%.py
pause

Save them as .bat and follow the instructions inside.

Question 21

Install the one you use most (3.3 in my case) over the top of the other. That’ll force IDLE to use the one you want.

Alternatively (from the python3.3 README):

Installing multiple versions

On Unix and Mac systems if you intend to install multiple versions of Python using the same installation prefix (–prefix argument to the configure script) you must take care that your primary python executable is not overwritten by the installation of a different version. All files and directories installed using “make altinstall” contain the major and minor version and can thus live side-by-side. “make install” also creates ${prefix}/bin/python3 which refers to ${prefix}/bin/pythonX.Y. If you intend to install multiple versions using the same prefix you must decide which version (if any) is your “primary” version. Install that version using “make install”. Install all other versions using “make altinstall”.

For example, if you want to install Python 2.6, 2.7 and 3.3 with 2.7 being the primary version, you would execute “make install” in your 2.7 build directory and “make altinstall” in the others.

Question 22

I just had to install them. Then I used the free (and portable) soft at http://defaultprogramseditor.com/ under “File type settings”/”Context menu”/search:”py”, chose .py file and added an ‘open’ command for the 2 IDLE by copying the existant command named ‘open with IDLE, changing names to IDLE 3.4.1/2.7.8, and remplacing the files numbers of their respective versions in the program path. Now I have just to right click the .py file and chose which IDLE I want to use. Can do the same with direct interpreters if you prefer.

Question 23

Only Works if your running your code in your Python IDE

I have both Python 2.7 and Python 3.3 installed on my windows operating system. If I try to launch a file, it will usually open up on the python 2.7 IDE. How I solved this issue, was when I choose to run my code on python 3.3, I open up python 3.3 IDLE(Python GUI), select file, open my file with the IDLE and save it. Then when I run my code, it runs to the IDLE that I currently opened it with. It works vice versa with 2.7.

Question 24

I have installed both python 2.7.13 and python 3.6.1 on windows 10pro and I was getting the same “Fatal error” when I tried pip2 or pip3.

What I did to correct this was to go to the location of python.exe for python 2 and python 3 files and create a copy of each, I then renamed each copy to python2.exe and python3.exe depending on the python version in the installation folder. I therefore had in each python installation folder both a python.exe file and a python2.exe or python3.exe depending on the python version.

This resolved my problem when I typed either pip2 or pip3.

Question 25

If you can’t get anything else to work, open an interpreter in whichever version you choose (I prefer using iPython) and:

import subprocess

subprocess.call('python script.py -flags')

This uses whichever python version you are currently operating under. Works fine for a single script, but will quickly get out of hand if there are lots of scripts you run, in which case you can always make a batch file with all of these calls inside. Not the most elegant answer, but it works.

Is there a way to make aliases for different python version a la Linux?

Python 实用宝典

标签归档：backwards-compatibility

Python的字符串格式化的许多方式-较旧的（即将被淘汰）吗？

问题：Python的字符串格式化的许多方式-较旧的（即将被淘汰）吗？

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See also

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向后兼容

Backwards Compatibility

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如何在Windows中同时安装Python 2.x和Python 3.x

问题：如何在Windows中同时安装Python 2.x和Python 3.x

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安装多个版本

Installing multiple versions

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