# pythons
Select the Python interpreter for a folder and build the environment for
its child processes.
[`use_app`](https://vedicreader.github.io/kunda/pythons.html#use_app)
sets the application name and environment-variable prefix used by Kunda
defaults.
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### app_env
``` python
def app_env(
name, default:NoneType=None
):
```
*What the environment holds for `env_name(name)`, asked now rather than
at import.*
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### env_name
``` python
def env_name(
name
):
```
*`name` under the host’s prefix, so `MAX_KERNELS` is `KUNDA_MAX_KERNELS`
by default.*
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### app_name
``` python
def app_name():
```
*What the host calls itself: `kunda`, until
[`use_app`](https://vedicreader.github.io/kunda/pythons.html#use_app)
says otherwise.*
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### use_app
``` python
def use_app(
name:str='kunda', env_prefix:str='KUNDA_'
):
```
*Name the host application, so what kunda reads and what it says are
spelled the host’s way.*
`VENV_PYTHONS` lists common virtual-environment interpreter paths in
lookup order.
``` python
VENV_PYTHONS[:4]
```
('.venv/bin/python',
'.venv/Scripts/python.exe',
'venv/bin/python',
'venv/Scripts/python.exe')
`BUNDLE_ONLY` lists interpreter variables inherited from a frozen
application.
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### clean_env
``` python
def clean_env():
```
*This process’s environment, safe to hand to a child.*
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### strip_bundle
``` python
def strip_bundle(
env, frozen:NoneType=None
):
```
*Strip py2app/py2exe bundle vars from env; no-op outside a bundle.*
[`strip_bundle`](https://vedicreader.github.io/kunda/pythons.html#strip_bundle)
removes frozen-application interpreter variables. Other environments are
unchanged.
``` python
env = {'PYTHONHOME': '/App.app/Contents/Resources', 'PATH': '/usr/bin', 'HOME': '/Users/me'}
strip_bundle(dict(env), frozen=True)
```
{'PATH': '/usr/bin', 'HOME': '/Users/me', 'PYTHONUTF8': '1'}
The returned environment enables UTF-8 mode.
``` python
test_eq(strip_bundle(dict(env), frozen=False), env) # nothing claimed, nothing changed
out = strip_bundle(dict(env), frozen=True)
assert not (set(BUNDLE_ONLY) & set(out)), 'every redirection name is gone'
test_eq(out['PATH'], '/usr/bin') # and nothing else is touched
test_eq(out['PYTHONUTF8'], '1')
```
[`clean_env`](https://vedicreader.github.io/kunda/pythons.html#clean_env)
applies
[`strip_bundle`](https://vedicreader.github.io/kunda/pythons.html#strip_bundle)
to the current process environment.
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### venv_env
``` python
def venv_env(
python:NoneType=None, env:NoneType=None
):
```
*`env` (this process’s, by default) with `python`’s virtual environment
in front of it.*
[`venv_env`](https://vedicreader.github.io/kunda/pythons.html#venv_env)
puts the selected virtual environment first on `PATH` and sets
`VIRTUAL_ENV`.
``` python
e = venv_env('/repo/.venv/bin/python', env={'PATH': '/usr/bin'})
e['VIRTUAL_ENV'], e['PATH']
```
('/repo/.venv', '/repo/.venv/bin:/usr/bin')
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### nearest_marked
``` python
def nearest_marked(
start, markers, stop:NoneType=None
):
```
*Walk up from `start` to `stop`, return (dir, marker) for the first dir
holding a marker.*
[`nearest_marked`](https://vedicreader.github.io/kunda/pythons.html#nearest_marked)
walks from `start` toward its parents. It returns the first directory
containing one of `markers`.
``` python
d, hit = nearest_marked(root/'repo'/'src'/'pkg', ['.git'])
d.name, hit.name
```
('repo', '.git')
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### nearest_python
``` python
def nearest_python(
start, stop:NoneType=None
):
```
*The nearest conventional venv interpreter at or above `start`, not
searched past `stop`.*
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### project_python
``` python
def project_python(
roots:tuple=()
):
```
*The first conventional project-local virtualenv interpreter among
`roots`, or None.*
The lookup helpers use the same marker and path rules.
``` python
outer = mkvenv(root/'repo')
nearest_python(root/'repo'/'src'/'pkg').replace(str(root), '/proj') # the temp root stands in for a checkout
```
'/proj/repo/.venv/bin/python'
The nearest virtual environment wins. A nested checkout can use its own
interpreter.
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### python_for
``` python
def python_for(
cwd:NoneType=None, stop:NoneType=None, roots:tuple=(), default:NoneType=None
):
```
*Pick interpreter for `cwd`: walk up to `stop`, then `default`, then
first venv in `roots`; None means use current.*
[`python_for`](https://vedicreader.github.io/kunda/pythons.html#python_for)
returns the interpreter used for processes started from a folder. `stop`
limits the parent search.
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### find_pythons
``` python
def find_pythons(
roots:tuple=(), current:NoneType=None, this_label:str='this one'
):
```
*Kernel launch options: this interpreter (`current`) and all reachable
venvs.`current` is resolved by
[`python_for`](https://vedicreader.github.io/kunda/pythons.html#python_for)
and may be deeper than standard search.Listing enables picker
selection.*
[`find_pythons`](https://vedicreader.github.io/kunda/pythons.html#find_pythons)
returns interpreters for a picker. It removes duplicate paths.
``` python
rows = find_pythons([root/'repo'], current=str(inner))
[r['label'] for r in rows]
```
['this one', 'src/.venv', 'repo/.venv', 'python3 on PATH', 'python on PATH']