Add entry point for python3 command

[hmaarrfk]

Issue:

On ubuntu, there is an entry point for both python3 and python3.

On windows, python3 doesn't exist.

Not sure about osx.

Could windows be added an entry point for the command python3 so that we can reach a broader audiance with "fool proof" instructions.

Ubuntu

Windows: (I deleted things in my roaming path between the two commands)

Environment (conda list):

``` $ conda list # packages in environment at C:\Users\mark\miniconda3: # # Name Version Build Channel anaconda-client 1.7.2 py_0 conda-forge attrs 19.3.0 py_0 conda-forge beautifulsoup4 4.9.1 py37hc8dfbb8_0 conda-forge blinker 1.4 py_1 conda-forge boolean.py 3.7 py_0 conda-forge brotlipy 0.7.0 py37h4ab8f01_1000 conda-forge bzip2 1.0.8 hfa6e2cd_2 conda-forge ca-certificates 2020.4.5.1 hecc5488_0 conda-forge certifi 2020.4.5.1 py37hc8dfbb8_0 conda-forge cffi 1.14.0 py37ha419a9e_0 conda-forge chardet 3.0.4 py37hc8dfbb8_1006 conda-forge clyent 1.2.2 py_1 conda-forge conda 4.8.3 py37hc8dfbb8_1 conda-forge conda-build 3.19.2 py37hc8dfbb8_2 conda-forge conda-forge-pinning 2020.05.25.23.01.15 0 conda-forge conda-package-handling 1.6.0 py37h702c6c1_2 conda-forge conda-smithy 3.7.3 py_0 conda-forge console_shortcut 0.1.1 4 cryptography 2.9.2 py37h26f1ce3_0 conda-forge decorator 4.4.2 py_0 conda-forge deprecated 1.2.10 pyh9f0ad1d_0 conda-forge filelock 3.0.12 pyh9f0ad1d_0 conda-forge git 2.26.2 0 conda-forge gitdb 4.0.5 py_0 conda-forge gitpython 3.1.2 py_0 conda-forge glob2 0.7 py_0 conda-forge idna 2.9 py_1 conda-forge importlib-metadata 1.6.0 py37hc8dfbb8_0 conda-forge importlib_metadata 1.6.0 0 conda-forge ipython_genutils 0.2.0 py_1 conda-forge isodate 0.6.0 py_1 conda-forge jinja2 2.11.2 pyh9f0ad1d_0 conda-forge jsonschema 3.2.0 py37hc8dfbb8_1 conda-forge jupyter_core 4.6.3 py37hc8dfbb8_1 conda-forge krb5 1.17.1 hdd46e55_0 conda-forge libarchive 3.3.3 h0c0e0cf_1008 conda-forge libcurl 7.69.1 h1dcc11c_0 conda-forge libiconv 1.15 hfa6e2cd_1006 conda-forge liblief 0.10.1 ha925a31_0 libsolv 0.7.13 h7bd577a_3 conda-forge libssh2 1.9.0 h3235a2c_2 conda-forge libxml2 2.9.10 h5d81f1c_1 conda-forge license-expression 1.2 py_0 conda-forge lz4-c 1.9.2 h62dcd97_1 conda-forge lzo 2.10 hfa6e2cd_1000 conda-forge m2-msys2-runtime 2.5.0.17080.65c939c 3 m2-patch 2.7.5 2 m2w64-gcc-libgfortran 5.3.0 6 m2w64-gcc-libs 5.3.0 7 m2w64-gcc-libs-core 5.3.0 7 m2w64-gmp 6.1.0 2 m2w64-libwinpthread-git 5.0.0.4634.697f757 2 mamba 0.2.12 py37h9da7104_0 conda-forge markupsafe 1.1.1 py37h8055547_1 conda-forge menuinst 1.4.16 py37_0 conda-forge msrest 0.6.10 py_0 conda-forge msys2-conda-epoch 20160418 1 nbformat 5.0.6 py_0 conda-forge oauthlib 3.0.1 py_0 conda-forge openssl 1.1.1g he774522_0 conda-forge pip 20.1.1 pyh9f0ad1d_0 conda-forge pkginfo 1.5.0.1 py_0 conda-forge powershell_shortcut 0.0.1 3 psutil 5.7.0 py37h8055547_1 conda-forge py-lief 0.10.1 py37ha925a31_0 pycosat 0.6.3 py37h8055547_1004 conda-forge pycparser 2.20 py_0 conda-forge pycrypto 2.6.1 py37h8055547_1004 conda-forge pygithub 1.51 py_0 conda-forge pyjwt 1.7.1 py_0 conda-forge pyopenssl 19.1.0 py_1 conda-forge pyrsistent 0.16.0 py37h8055547_0 conda-forge pysocks 1.7.1 py37hc8dfbb8_1 conda-forge python 3.7.6 h60c2a47_5_cpython conda-forge python-dateutil 2.8.1 py_0 conda-forge python-libarchive-c 2.9 py37_0 conda-forge python_abi 3.7 1_cp37m conda-forge pytz 2020.1 pyh9f0ad1d_0 conda-forge pywin32 227 py37hfa6e2cd_0 conda-forge pyyaml 5.3.1 py37h8055547_0 conda-forge requests 2.23.0 pyh8c360ce_2 conda-forge requests-oauthlib 1.2.0 py_0 conda-forge ripgrep 12.1.0 h301d43c_0 conda-forge ruamel.yaml 0.16.6 py37h8055547_1 conda-forge ruamel.yaml.clib 0.2.0 py37h8055547_1 conda-forge ruamel_yaml 0.15.80 py37h8055547_1001 conda-forge scrypt 0.8.15 py37h90766aa_0 conda-forge setuptools 46.4.0 py37hc8dfbb8_0 conda-forge six 1.15.0 pyh9f0ad1d_0 conda-forge smmap 3.0.4 pyh9f0ad1d_0 conda-forge soupsieve 2.0.1 py37hc8dfbb8_0 conda-forge sqlite 3.30.1 hfa6e2cd_0 conda-forge tk 8.6.10 hfa6e2cd_0 conda-forge toolz 0.10.0 py_0 conda-forge tqdm 4.46.0 pyh9f0ad1d_0 conda-forge traitlets 4.3.3 py37hc8dfbb8_1 conda-forge urllib3 1.25.9 py_0 conda-forge vc 14.1 h869be7e_1 conda-forge vs2015_runtime 14.16.27012 h30e32a0_2 conda-forge vsts-python-api 0.1.22 py_0 conda-forge wheel 0.34.2 py_1 conda-forge win_inet_pton 1.1.0 py37_0 conda-forge wincertstore 0.2 py37_1003 conda-forge wrapt 1.12.1 py37h8055547_1 conda-forge xz 5.2.5 h2fa13f4_0 conda-forge yaml 0.2.4 he774522_0 conda-forge zipp 3.1.0 py_0 conda-forge zlib 1.2.11 h2fa13f4_1006 conda-forge zstd 1.4.4 h9f78265_3 conda-forge ```

Details about conda and system ( conda info ):

``` (base) C:\Users\mark>conda info active environment : base active env location : C:\Users\mark\miniconda3 shell level : 1 user config file : C:\Users\mark\.condarc populated config files : C:\Users\mark\.condarc conda version : 4.8.3 conda-build version : 3.19.2 python version : 3.7.6.final.0 virtual packages : __cuda=10.1 base environment : C:\Users\mark\miniconda3 (writable) channel URLs : https://conda.anaconda.org/ramonaoptics/win-64 https://conda.anaconda.org/ramonaoptics/noarch https://conda.anaconda.org/conda-forge/win-64 https://conda.anaconda.org/conda-forge/noarch https://repo.anaconda.com/pkgs/main/win-64 https://repo.anaconda.com/pkgs/main/noarch https://repo.anaconda.com/pkgs/r/win-64 https://repo.anaconda.com/pkgs/r/noarch https://repo.anaconda.com/pkgs/msys2/win-64 https://repo.anaconda.com/pkgs/msys2/noarch package cache : C:\Users\mark\miniconda3\pkgs C:\Users\mark\.conda\pkgs C:\Users\mark\AppData\Local\conda\conda\pkgs envs directories : C:\Users\mark\miniconda3\envs C:\Users\mark\.conda\envs C:\Users\mark\AppData\Local\conda\conda\envs platform : win-64 user-agent : conda/4.8.3 requests/2.23.0 CPython/3.7.6 Windows/10 Windows/10.0.18362 administrator : False netrc file : None offline mode : False ```

[hmaarrfk]

@conda-forge/core any thoughts on this?

We were able to move a pip3 package through, but I know this one may be a little more contentious.

[isuruf]

How do you propose we build a python3.exe? copy python.exe to python3.exe? That's a waste of space.

[hmaarrfk]

Actually, I hadn't thought of that, and that sounds like a great easy idea!!!

100kB copy seems well worth it to me.

Detailed motivation

In the cross referenced issue, I'm trying to get people going with `scikit-image` as fast as possible, and unfortunately, I am forced to prefix instructions with: ``` **Finding your python command** To install ``scikit-image`` with `pip `_, you may use the following commands from a command prompt. Note that this command prompt is not the python command prompt, but, depending on your operating system, this is called: - A linux terminal window - A MacOSX terminal window - A Windows Command Prompt In that windows, please type: .. code-block:: sh # For linux or mac which python3 # Linux and Mac might not have the command python which python # for windows where python # Windows might not have the command python3 where python3 To ensure you know which python executable you are using. For Linux and mac, we found that depending on their operating system, users should use: - For MacOSX: ``python3`` - For Linux: ``python3`` - For Windows: ``python`` (Notice the lack of 3) Once you have found your python command, use .. code-blocks:: sh python --version python3 --version To ensure the version is at least Python 3.6. If not, you may not be able to install the most updated version of scikit-image. However, this should not stop you from installing an older version and get you going with your image processing project. We simply suggest you check the installed version of ``scikit-image`` with the code snippet included at the top of this guide so that you can refer to the appropriate documentation version. ```

[tony-p]

Any movement on this? There are conda packages (for me robostack/humble) that have scripts with python3 shebangs in the build chain which isn't just about changing user behaviour. Now I am manually adding a symlink, but that doesn't make for very portable environments

CMake Error at D:/.../share/cmake-3.27/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find Python3 (missing: Python3_NumPy_INCLUDE_DIRS NumPy)

[tony-p]

@isuruf @jjhelmus @xhochy I have made PRs to make a python3 alias via copy. Can they please be merged? The file that is copied is <100kb which I think is ok to get rid of this problem

[isuruf]

I still don't see why this is needed.

[hmaarrfk]

For original source writers,

It is very difficult to write "cross os" installation and running instructions.

1. If on ubuntu 20.04 please use the command python3.
2. if you installed windows please use the command python
3. if you installed with this method.....

The list kinda goes on.

This really helps first time started get going with python

[isuruf]

So, you want to complicate the instructions with the following?

1. If on ubuntu 20.04 please use the command python3.
2. If you installed on windows with conda, use the command python3
3. if you installed windows but not conda, please use the command python
4. if you installed with this method.....

[hmaarrfk]

I would rather say:

1. Make sure you have a working installation of python. We recommend using the installers from conda-forge.
2. python3.

In 2020, when I opened the issue, python2 was a real thing. so users accidentally having python2 was a real problem.

I don't think there is a good "solution" to these problems, but I think that adding a python3 command is a good step forward. At the very least, it would add pressure on upstream cpython to add a python3 command on their default windows installer.

[tony-p]

The fact that this behaviour is different between linux and windows to me seems like enough of an argument to say the python3 alias should be included.

Further there are packaged tools/scripts that use a python3 shebang, these simply do not work with the base python package (or worse call the global install)

Why I pinged again today is I had to python myenv/Library/tools/the_tool_I_want/script.py because the script used a python3 shebang. This defies the whole point of having an environment

[tony-p]

At the very least, it would add pressure on upstream cpython to add a python3 command on their default windows installer.

In Windows they use the pylauncher (what causes more headaches with conda environments) which emulates the python3 aliases, but only for global python installs

[isuruf]

Can you both please comment on https://github.com/python/cpython/issues/99185 first?

[hmaarrfk]

ok thank you for the constructive comment.

I somewhat remember clashes between name.dll and name.exe.

I'm going to try to test the new entry points before suggesting. I've been hit by similar bugs in the past in other packages.