Since Python 3.3, the following classes have merged into OSError. They
remain as aliases for backward compatibility.
- EnvironmentError
- IOError
- WindowsError
https://docs.python.org/3/library/exceptions.html#OSError
Python 3 also has subclasses of OSError to help identify more specific
errors. For example, FileNotFoundError. This allows simplifying some
except blocks.
Use pyupgrade to convert simple string formatting to use f-string
syntax. pyupgrade is intentionally timid and will not create an f-string
if it would make the expression longer or if the substitution parameters
are anything but simple names or dotted names.
This is done by catching InstallationError from the underlying
distribution preparation logic. There are three cases to catch:
1. Candidates from indexes. These are simply ignored since we can
potentially satisfy the requirement with other candidates.
2. Candidates from URLs with a dist name (PEP 508 or #egg=). A new
UnsatisfiableRequirement class is introduced to represent this; it is
like an ExplicitRequirement without an underlying candidate. As the
name suggests, an instance of this can never be satisfied, and will
cause eventual backtracking.
3. Candidates from URLs without a dist name. This is only possible for
top-level user requirements, and no recourse is possible for them. So
we error out eagerly.
The InstallationError raised during distribution preparation is cached
in the factory, like successfully prepared candidates, since we don't
want to repeatedly try to build a candidate if we already know it'd
fail. Plus pip's preparation logic also does not allow packages to be
built multiple times anyway.
These tests use the script fixture which as a 30s setup time on my
machine. This KILLS productivity when trying to run unit tests as part
of a feedback loop during development.
find_matches() is modified to return a special type that implements
the sequence protocol (instead of a plain list). This special sequence
type tries to use the installed candidate as the first element if
possible, and only access indexes when the installed candidate is
considered unsatisfactory.