fix: revert "Map also empty dictionaries to file" (#1629)

* Revert "Map also empty dictionaries to file"

This reverts commit f54428f (#1347),
which was a big regression in performance and file sizes when using
dynamic_context, and is no longer needed for #972 as of
4cc3292 (#1538).

Fixes #1586.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

* docs: details of the fix in pull #1629, fixing #1586.

---------

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

CHANGES.rst

@@ -20,7 +20,14 @@ development at the same time, such as 4.5.x and 5.0.
 Unreleased
 ----------
 
-Nothing yet.
+- Fix: reverted a `change from 6.4.3 <pull 1347_>`_ that helped Cython, but
+  also increased the size of data files when using dynamic contexts, as
+  described in the now-fixed `issue 1586`_. The problem is now avoided due to a
+  recent change (`issue 1538`_).  Thanks to `Anders Kaseorg <pull 1629_>`_
+  and David Szotten for persisting with problem reports and detailed diagnoses.
+
+.. _issue 1586: https://github.com/nedbat/coveragepy/issues/1586
+.. _pull 1629: https://github.com/nedbat/coveragepy/pull/1629
 
 
 .. scriv-start-here

CONTRIBUTORS.txt

@@ -16,6 +16,7 @@ Alexander Todorov
 Alexander Walters
 Alpha Chen
 Ammar Askar
+Anders Kaseorg
 Andrew Hoos
 Anthony Sottile
 Arcadiy Ivanov

coverage/collector.py

@@ -456,7 +456,7 @@ def mapped_file_dict(self, d: Mapping[str, T]) -> Dict[str, T]:
             assert isinstance(runtime_err, Exception)
             raise runtime_err
 
-        return {self.cached_mapped_file(k): v for k, v in items}
+        return {self.cached_mapped_file(k): v for k, v in items if v}
 
     def plugin_was_disabled(self, plugin: CoveragePlugin) -> None:
         """Record that `plugin` was disabled during the run."""