strip trailing + from version number of local Python builds (#1771)

(This PR message is mostly copied from the comment in the code.)

For local builds of Python, at time of writing, the version numbers end
with
a `+`. This makes the version non-PEP-440 compatible since a `+`
indicates
the start of a local segment which must be non-empty. Thus, `uv` chokes
on it
and [spits out an error][1] when trying to create a venv using a "local"
build
of Python. Arguably, the right fix for this is for [CPython to use a
PEP-440
compatible version number][2].

However, as a work-around for now, [as suggested by pradyunsg][3] as one
possible direction forward, we strip the `+`.

This fix does unfortunately mean that one [cannot specify a Python
version
constraint that specifically selects a local version][4]. But at the
time of
writing, it seems reasonable to block such functionality on this being
fixed
upstream (in some way).

Another alternative would be to treat such invalid versions as strings
(which
is what PEP-508 suggests), but this leads to undesirable behavior in
this
case. For example, let's say you have a Python constraint of `>=3.9.1`
and
a local build of Python with a version `3.11.1+`. Using string
comparisons
would mean the constraint wouldn't be satisfied:

    >>> "3.9.1" < "3.11.1+"
    False

So in the end, we just strip the trailing `+`, as was done in the days
of old
for [legacy version numbers][5].

I tested this fix by manually confirming that

    uv venv --python local/python

failed before it and succeeded after it.

Fixes #1357

[1]: #1357
[2]: python/cpython#99968
[3]:
pypa/packaging#678 (comment)
[4]: #1357 (comment)
[5]:
https://github.com/pypa/packaging/blob/085ff41692b687ae5b0772a55615b69a5b677be9/packaging/version.py#L168-L193

crates/uv-interpreter/src/get_interpreter_info.py

@@ -19,6 +19,43 @@ def format_full_version(info):
 else:
     implementation_version = "0"
     implementation_name = ""
+
+python_full_version = platform.python_version()
+# For local builds of Python, at time of writing, the version numbers end with
+# a `+`. This makes the version non-PEP-440 compatible since a `+` indicates
+# the start of a local segment which must be non-empty. Thus, `uv` chokes on it
+# and spits out an error[1] when trying to create a venv using a "local" build
+# of Python. Arguably, the right fix for this is for CPython to use a PEP-440
+# compatible version number[2].
+#
+# However, as a work-around for now, as suggested by pradyunsg[3] as one
+# possible direction forward, we strip the `+`.
+#
+# This fix does unfortunately mean that one cannot specify a Python version
+# constraint that specifically selects a local version[4]. But at the time of
+# writing, it seems reasonable to block such functionality on this being fixed
+# upstream (in some way).
+#
+# Another alternative would be to treat such invalid versions as strings (which
+# is what PEP-508 suggests), but this leads to undesirable behavior in this
+# case. For example, let's say you have a Python constraint of `>=3.9.1` and
+# a local build of Python with a version `3.11.1+`. Using string comparisons
+# would mean the constraint wouldn't be satisfied:
+#
+#     >>> "3.9.1" < "3.11.1+"
+#     False
+#
+# So in the end, we just strip the trailing `+`, as was done in the days of old
+# for legacy version numbers[5].
+#
+# [1]: https://github.com/astral-sh/uv/issues/1357
+# [2]: https://github.com/python/cpython/issues/99968
+# [3]: https://github.com/pypa/packaging/issues/678#issuecomment-1436033646
+# [4]: https://github.com/astral-sh/uv/issues/1357#issuecomment-1947645243
+# [5]: https://github.com/pypa/packaging/blob/085ff41692b687ae5b0772a55615b69a5b677be9/packaging/version.py#L168-L193
+if len(python_full_version) > 0 and python_full_version[-1] == '+':
+    python_full_version = python_full_version[:-1]
+
 markers = {
     "implementation_name": implementation_name,
     "implementation_version": implementation_version,
@@ -28,7 +65,7 @@ def format_full_version(info):
     "platform_release": platform.release(),
     "platform_system": platform.system(),
     "platform_version": platform.version(),
-    "python_full_version": platform.python_version(),
+    "python_full_version": python_full_version,
     "python_version": ".".join(platform.python_version_tuple()[:2]),
     "sys_platform": sys.platform,
 }