On Windows, build scripts should prefer using python.exe from an active virtual env

[taleinat]

BPO	38809
Nosy	@pfmoore, @taleinat, @tjguk, @zware, @zooba, @miss-islington
PRs	bpo-38809: Windows build scripts use python.exe from virtual envs #17164 [3.8] bpo-38809: Windows build scripts use python.exe from virtual envs (GH-17164) #17237 [3.7] bpo-38809: Windows build scripts use python.exe from virtual envs (GH-17164) #17238

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-11-18.17:54:12.228>
created_at = <Date 2019-11-15.07:23:46.875>
labels = ['build', '3.8', '3.9', 'type-feature', '3.7', 'OS-windows']
title = 'On Windows, build scripts should prefer using python.exe from an active virtual env'
updated_at = <Date 2019-11-18.17:54:12.227>
user = 'https://github.com/taleinat'

bugs.python.org fields:

activity = <Date 2019-11-18.17:54:12.227>
actor = 'taleinat'
assignee = 'none'
closed = True
closed_date = <Date 2019-11-18.17:54:12.228>
closer = 'taleinat'
components = ['Build', 'Windows']
creation = <Date 2019-11-15.07:23:46.875>
creator = 'taleinat'
dependencies = []
files = []
hgrepos = []
issue_num = 38809
keywords = ['patch']
message_count = 4.0
messages = ['356649', '356889', '356890', '356891']
nosy_count = 6.0
nosy_names = ['paul.moore', 'taleinat', 'tim.golden', 'zach.ware', 'steve.dower', 'miss-islington']
pr_nums = ['17164', '17237', '17238']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38809'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[taleinat]

On windows several .bat scripts call find_python.bat to find an appropriate python.exe. find_python.bat has no specific support for virtual envs, and usually ends up calling py.exe to find python.exe. Due to virtual envs not including a py.exe, this effectively ignores having an activated virtual env.

It is currently possible to build with the specific python that one wants by setting the PYTHON env var. However, this is a simplistic and non-standard solution compared to virtual envs.

IMO when there is an active virtual env, these scripts should certainly use python.exe from it!

[miss-islington]

New changeset ee703cb by Miss Islington (bot) (Tal Einat) in branch 'master':
bpo-38809: Windows build scripts use python.exe from virtual envs (GH-17164)
ee703cb

[miss-islington]

New changeset 2b928d9 by Miss Islington (bot) in branch '3.7':
bpo-38809: Windows build scripts use python.exe from virtual envs (GH-17164)
2b928d9

[miss-islington]

New changeset cbbf109 by Miss Islington (bot) in branch '3.8':
bpo-38809: Windows build scripts use python.exe from virtual envs (GH-17164)
cbbf109