new command: pdm use

.gitignore

@@ -131,3 +131,4 @@ dmypy.json
 caches/
 .idea/
 __pypackages__
+.pdm.toml

pdm/cli/actions.py

 import itertools
+import os
 import shutil
 from typing import Dict, Iterable, Optional, Sequence
 
@@ -6,10 +7,11 @@ from pkg_resources import safe_name
 
 import click
 import halo
+import pythonfinder
 import tomlkit
 from pdm.builders import SdistBuilder, WheelBuilder
 from pdm.context import context
-from pdm.exceptions import ProjectError
+from pdm.exceptions import NoPythonVersion, ProjectError
 from pdm.installers import Synchronizer, format_dist
 from pdm.models.candidates import Candidate, identify
 from pdm.models.requirements import parse_requirement, strip_extras
@@ -17,6 +19,7 @@ from pdm.models.specifiers import bump_version, get_specifier
 from pdm.project import Project
 from pdm.resolver import BaseProvider, EagerUpdateProvider, ReusePinProvider, resolve
 from pdm.resolver.reporters import SpinnerReporter
+from pdm.utils import get_python_version
 
 
 def format_lockfile(mapping, fetched_dependencies, summary_collection):
@@ -386,3 +389,33 @@ def do_init(
         project._pyproject.setdefault("tool", {})["pdm"] = data["tool"]["pdm"]
         project._pyproject["build-system"] = data["build-system"]
     project.write_pyproject()
+
+
+def do_use(project: Project, python: str) -> None:
+    """Use the specified python version and save in project config.
+    The python can be a version string or interpreter path.
+    """
+    if os.path.isabs(python):
+        python_path = python
+    else:
+        python_path = shutil.which(python)
+        if not python_path:
+            finder = pythonfinder.Finder()
+            try:
+                python_path = finder.find_python_version(python).path.as_posix()
+            except AttributeError:
+                raise NoPythonVersion(f"Python {python} is not found on the system.")
+
+    python_version = ".".join(map(str, get_python_version(python_path)))
+    if not project.python_requires.contains(python_version):
+        raise NoPythonVersion(
+            "The target Python version {} doesn't satisfy "
+            "the Python requirement: {}".format(python_version, project.python_requires)
+        )
+    context.io.echo(
+        "Using Python interpreter: {} ({})".format(
+            context.io.green(python_path), python_version
+        )
+    )
+    project.config["python"] = python_path
+    project.config.save_config()

pdm/cli/commands.py

@@ -264,3 +264,11 @@ def init(project):
     author = click.prompt(f"Author name", default=git_user)
     email = click.prompt(f"Author email", default=git_email)
     actions.do_init(project, name, version, license, author, email)
+
+
+@cli.command()
+@click.argument("python")
+@pass_project
+def use(project, python):
+    """Use the given python version as base interpreter."""
+    actions.do_use(project, python)

pdm/models/environment.py

@@ -84,6 +84,7 @@ class Environment:
     @cached_property
     def python_executable(self) -> str:
         """Get the Python interpreter path."""
+        path = None
         if self.config["python"]:
             path = self.config["python"]
             try:
@@ -96,11 +97,23 @@ class Environment:
             for python in finder.find_all_python_versions():
                 version = ".".join(map(str, get_python_version(python.path.as_posix())))
                 if self.python_requires.contains(version):
-                    return python.path.as_posix()
+                    path = python.path.as_posix()
             if self.python_requires.contains(".".join(map(str, sys.version_info[:3]))):
-                return sys.executable
+                path = sys.executable
+            if path:
+                python_version = ".".join(map(str, get_python_version(path)))
+                context.io.echo(
+                    "Using Python interpreter: {} ({})".format(
+                        context.io.green(path), python_version
+                    )
+                )
+                self.config["python"] = path
+                self.config.save_config()
+                return path
         raise NoPythonVersion(
-            "No python matching {} is found on the system.".format(self.python_requires)
+            "No Python that satisfies {} is found on the system.".format(
+                self.python_requires
+            )
         )
 
     def get_paths(self) -> Dict[str, str]:

pdm/project/config.py

@@ -56,7 +56,7 @@ class Config(MutableMapping):
                 temp = temp.setdefault(part, {})
             temp[last] = value
 
-        with file_path.open(encoding="utf-8") as fp:
+        with file_path.open("w", encoding="utf-8") as fp:
             fp.write(tomlkit.dumps(toml_data))
         self._dirty.clear()
 

tests/cli/test_cli.py

 import functools
 import os
+import shutil
 
 import pytest
 from click.testing import CliRunner
@@ -91,3 +92,19 @@ def test_uncaught_error(invoke, mocker):
 
     result = invoke(["list", "-v"])
     assert isinstance(result.exception, RuntimeError)
+
+
+def test_use_command(project, invoke):
+    python_path = shutil.which("python")
+    result = invoke(["use", "python"], obj=project)
+    assert result.exit_code == 0
+    config_content = project.root.joinpath(".pdm.toml").read_text()
+    assert python_path in config_content
+
+    result = invoke(["use", python_path], obj=project)
+    assert result.exit_code == 0
+
+    project.tool_settings["python_requires"] = ">=3.6"
+    project.write_pyproject()
+    result = invoke(["use", "2.7"], obj=project)
+    assert result.exit_code == 1