main: early exit on non-existent script files

When trying to run PyInstaller against non-existent script file(s),
exit with error message immediately, before writing the spec file
and trying to build it.

This prevents us from overwriting an existing (and customized) .spec
file if user makes a typo in the .spec file's suffix when trying to
build it, for example, `pyinstaller program.cpes`. It also prevents
creation of a .spec file when `pyinstaller program.py` is accidentally
ran from a directory that does not contain the script (for example,
due to failing to change the directory prior to running the command).

This applies only to the `pyinstaller` command (and the equivalent
`python -m PyInstaller` invocation); the `pyi-makespec` command is
still allowed to be called with non-existent script file(s) that
might be added later, before the actual build attempt with the
generated .spec file.

PyInstaller/__main__.py

@@ -197,6 +197,15 @@ def run(pyi_args: list | None = None, pyi_config: dict | None = None):
             )
             spec_file = args.filenames[0]
         else:
+            # Ensure that the given script files exist, before trying to generate the .spec file.
+            # This prevents us from overwriting an existing (and customized) .spec file if user makes a typo in the
+            # .spec file's suffix when trying to  build it, for example, `pyinstaller program.cpes` (see #8276).
+            # It also prevents creation of a .spec file when `pyinstaller program.py` is accidentally ran from a
+            # directory that does not contain the script (for example, due to failing to change the directory prior
+            # to running the command).
+            for filename in args.filenames:
+                if not os.path.isfile(filename):
+                    raise SystemExit(f"Script file {filename!r} does not exist.")
             spec_file = run_makespec(**vars(args))
 
         sys.argv = [spec_file, *spec_args]

news/8279.bugfix.rst

@@ -0,0 +1,5 @@
+When trying to run ``pyinstaller`` (or equivalent ``python -m PyInstaller``)
+against non-existing script file(s), exit immediately - without trying
+to write the .spec file and building it. This prevents us from overwriting
+an existing (and customized) .spec file if user makes a typo in the .spec
+file's suffix when trying to build it, for example, ``pyinstaller program.cpes``.