Repository: pre-commit/pre-commit (python)

Issue:
Support for conda as a language
It would be nice to have support for conda in `pre-commit` as a "language"/environment manager. This would enable setups where packages are installed through conda. The typical usage for this will be for Python and R setups (e.g. https://github.com/lorenzwalthert/precommit/issues/99) where conda is most popular but as `conda` is language independent.

The implementation would work as follows:

- A hook is expected to have a requirements.txt that is installed using `conda create -p <prefix> --file=requirements.txt`. I would not use the conda `environment.yml` as this already dictates the name of the environment.
- The hook is run using `conda run -p <prefix> <entry>`
- We will use the `conda` executable that is available on the `PATH`.

Note: I would volunteer to implement this once there are some 👍 that the proposal is sound.

Pull request description:
Support for conda as a language
Fixes #1204

## Example usage

`.pre-commit-hooks.yaml`:

```
- id: black-conda
  name: black-conda
  description: "Black: The uncompromising Python code formatter"
  entry: black
  language: conda
  require_serial: true
  types: [python]
```

`environment.yml`:

```
channels:
  - conda-forge
  - defaults
dependencies:
  - black=19.10b0
```

Patch:
diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 5b57e89..9d61eb6 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -22,6 +22,9 @@ jobs:
       COVERAGE_IGNORE_WINDOWS: '# pragma: windows no cover'
       TOX_TESTENV_PASSENV: COVERAGE_IGNORE_WINDOWS
       TEMP: C:\Temp  # remove when dropping python2
+    pre_test:
+    - powershell: Write-Host "##vso[task.prependpath]$env:CONDA\Scripts"
+      displayName: Add conda to PATH
 - template: job--python-tox.yml@asottile
   parameters:
     toxenvs: [py37]
diff --git a/pre_commit/languages/all.py b/pre_commit/languages/all.py
index 051656b..3d139d9 100644
--- a/pre_commit/languages/all.py
+++ b/pre_commit/languages/all.py
@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+from pre_commit.languages import conda
 from pre_commit.languages import docker
 from pre_commit.languages import docker_image
 from pre_commit.languages import fail
@@ -52,6 +53,7 @@ from pre_commit.languages import system
 #     """
 
 languages = {
+    'conda': conda,
     'docker': docker,
     'docker_image': docker_image,
     'fail': fail,
diff --git a/pre_commit/languages/conda.py b/pre_commit/languages/conda.py
new file mode 100644
index 0000000..a89d6c9
--- /dev/null
+++ b/pre_commit/languages/conda.py
@@ -0,0 +1,66 @@
+import contextlib
+import os
+
+from pre_commit.envcontext import envcontext
+from pre_commit.envcontext import UNSET
+from pre_commit.envcontext import Var
+from pre_commit.languages import helpers
+from pre_commit.util import clean_path_on_failure
+from pre_commit.util import cmd_output_b
+
+ENVIRONMENT_DIR = 'conda'
+get_default_version = helpers.basic_get_default_version
+healthy = helpers.basic_healthy
+
+
+def get_env_patch(env):
+    # On non-windows systems executable live in $CONDA_PREFIX/bin, on Windows
+    # they can be in $CONDA_PREFIX/bin, $CONDA_PREFIX/Library/bin,
+    # $CONDA_PREFIX/Scripts and $CONDA_PREFIX. Whereas the latter only
+    # seems to be used for python.exe.
+    path = (os.path.join(env, 'bin'), os.pathsep, Var('PATH'))
+    if os.name == 'nt':  # pragma: no cover (platform specific)
+        path = (env, os.pathsep) + path
+        path = (os.path.join(env, 'Scripts'), os.pathsep) + path
+        path = (os.path.join(env, 'Library', 'bin'), os.pathsep) + path
+
+    return (
+        ('PYTHONHOME', UNSET),
+        ('VIRTUAL_ENV', UNSET),
+        ('CONDA_PREFIX', env),
+        ('PATH', path),
+    )
+
+
+@contextlib.contextmanager
+def in_env(prefix, language_version):
+    directory = helpers.environment_dir(ENVIRONMENT_DIR, language_version)
+    envdir = prefix.path(directory)
+    with envcontext(get_env_patch(envdir)):
+        yield
+
+
+def install_environment(prefix, version, additional_dependencies):
+    helpers.assert_version_default('conda', version)
+    directory = helpers.environment_dir(ENVIRONMENT_DIR, version)
+
+    env_dir = prefix.path(directory)
+    with clean_path_on_failure(env_dir):
+        cmd_output_b(
+            'conda', 'env', 'create', '-p', env_dir, '--file',
+            'environment.yml', cwd=prefix.prefix_dir,
+        )
+        if additional_dependencies:
+            cmd_output_b(
+                'conda', 'install', '-p', env_dir, *additional_dependencies,
+                cwd=prefix.prefix_dir
+            )
+
+
+def run_hook(hook, file_args, color):
+    # TODO: Some rare commands need to be run using `conda run` but mostly we
+    #       can run them withot which is much quicker and produces a better
+    #       output.
+    # cmd = ('conda', 'run', '-p', env_dir) + hook.cmd
+    with in_env(hook.prefix, hook.language_version):
+        return helpers.run_xargs(hook, hook.cmd, file_args, color=color)
diff --git a/pre_commit/resources/empty_template_environment.yml b/pre_commit/resources/empty_template_environment.yml
new file mode 100644
index 0000000..0f29f0c
--- /dev/null
+++ b/pre_commit/resources/empty_template_environment.yml
@@ -0,0 +1,9 @@
+channels:
+  - conda-forge
+  - defaults
+dependencies:
+  # This cannot be empty as otherwise no environment will be created.
+  # We're using openssl here as it is available on all system and will
+  # most likely be always installed anyways.
+  # See https://github.com/conda/conda/issues/9487
+  - openssl
diff --git a/pre_commit/store.py b/pre_commit/store.py
index 2f15924..d9b674b 100644
--- a/pre_commit/store.py
+++ b/pre_commit/store.py
@@ -173,7 +173,7 @@ class Store(object):
 
     LOCAL_RESOURCES = (
         'Cargo.toml', 'main.go', 'main.rs', '.npmignore', 'package.json',
-        'pre_commit_dummy_package.gemspec', 'setup.py',
+        'pre_commit_dummy_package.gemspec', 'setup.py', 'environment.yml',
     )
 
     def make_local(self, deps):


Test patch:
diff --git a/testing/resources/conda_hooks_repo/.pre-commit-hooks.yaml b/testing/resources/conda_hooks_repo/.pre-commit-hooks.yaml
new file mode 100644
index 0000000..a0d274c
--- /dev/null
+++ b/testing/resources/conda_hooks_repo/.pre-commit-hooks.yaml
@@ -0,0 +1,10 @@
+-   id: sys-exec
+    name: sys-exec
+    entry: python -c 'import os; import sys; print(sys.executable.split(os.path.sep)[-2]) if os.name == "nt" else print(sys.executable.split(os.path.sep)[-3])'
+    language: conda
+    files: \.py$
+-   id: additional-deps
+    name: additional-deps
+    entry: python
+    language: conda
+    files: \.py$
diff --git a/testing/resources/conda_hooks_repo/environment.yml b/testing/resources/conda_hooks_repo/environment.yml
new file mode 100644
index 0000000..e23c079
--- /dev/null
+++ b/testing/resources/conda_hooks_repo/environment.yml
@@ -0,0 +1,6 @@
+channels:
+  - conda-forge
+  - defaults
+dependencies:
+  - python
+  - pip
diff --git a/tests/repository_test.py b/tests/repository_test.py
index 85afa90..5f2ed1c 100644
--- a/tests/repository_test.py
+++ b/tests/repository_test.py
@@ -79,6 +79,46 @@ def _test_hook_repo(
     assert _norm_out(out) == expected
 
 
+def test_conda_hook(tempdir_factory, store):
+    _test_hook_repo(
+        tempdir_factory, store, 'conda_hooks_repo',
+        'sys-exec', [os.devnull],
+        b'conda-default\n',
+    )
+
+
+def test_conda_with_additional_dependencies_hook(tempdir_factory, store):
+    _test_hook_repo(
+        tempdir_factory, store, 'conda_hooks_repo',
+        'additional-deps', [os.devnull],
+        b'OK\n',
+        config_kwargs={
+            'hooks': [{
+                'id': 'additional-deps',
+                'args': ['-c', 'import mccabe; print("OK")'],
+                'additional_dependencies': ['mccabe'],
+            }],
+        },
+    )
+
+
+def test_local_conda_additional_dependencies(store):
+    config = {
+        'repo': 'local',
+        'hooks': [{
+            'id': 'local-conda',
+            'name': 'local-conda',
+            'entry': 'python',
+            'language': 'conda',
+            'args': ['-c', 'import mccabe; print("OK")'],
+            'additional_dependencies': ['mccabe'],
+        }],
+    }
+    ret, out = _get_hook(config, store, 'local-conda').run((), color=False)
+    assert ret == 0
+    assert _norm_out(out) == b'OK\n'
+
+
 def test_python_hook(tempdir_factory, store):
     _test_hook_repo(
         tempdir_factory, store, 'python_hooks_repo',