feat: --diff to show what failed --check

Author: nedbat

CHANGELOG.rst

@@ -21,6 +21,9 @@ These are changes to Cog over time.
 Unreleased
 ----------
 
+- Added a ``--diff`` option to show the diff of what changed to fail a
+  ``--check`` run.
+
 - Embedded code can change the current directory, cog will change back to the
   original directory when the code is done.
 

cogapp/cogapp.py

@@ -1,6 +1,7 @@
 """Cog content generation tool."""
 
 import copy
+import difflib
 import getopt
 import glob
 import io
@@ -50,6 +51,7 @@
     -z          The end-output marker can be omitted, and is assumed at eof.
     -v          Print the version of cog and exit.
     --check     Check that the files would not change if run again.
+    --diff      With --check, show a diff of what failed the check.
     --markers='START END END-OUTPUT'
                 The patterns surrounding cog inline instructions. Should
                 include three values separated by spaces, the start, end,
@@ -240,6 +242,7 @@ def __init__(self):
         self.prologue = ""
         self.print_output = False
         self.check = False
+        self.diff = False
 
     def __eq__(self, other):
         """Comparison operator for tests to use."""
@@ -262,6 +265,7 @@ def parse_args(self, argv):
                 "cdD:eI:n:o:rs:p:PUvw:xz",
                 [
                     "check",
+                    "diff",
                     "markers=",
                     "verbosity=",
                 ],
@@ -308,6 +312,8 @@ def parse_args(self, argv):
                 self.eof_can_be_end = True
             elif o == "--check":
                 self.check = True
+            elif o == "--diff":
+                self.diff = True
             elif o == "--markers":
                 self._parse_markers(a)
             elif o == "--verbosity":
@@ -335,6 +341,9 @@ def validate(self):
         if self.replace and self.output_name:
             raise CogUsageError("Can't use -o with -r (they are opposites)")
 
+        if self.diff and not self.check:
+            raise CogUsageError("Can't use --diff without --check")
+
 
 class Cog(Redirectable):
     """The Cog engine."""
@@ -692,6 +701,18 @@ def process_one_file(self, fname):
                         else:
                             assert self.options.check
                             self.check_failed = True
+                            if self.options.diff:
+                                old_lines = old_text.splitlines()
+                                new_lines = new_text.splitlines()
+                                diff = difflib.unified_diff(
+                                    old_lines,
+                                    new_lines,
+                                    fromfile=f"current {fname}",
+                                    tofile=f"changed {fname}",
+                                    lineterm="",
+                                )
+                                for diff_line in diff:
+                                    self.prout(diff_line)
                 finally:
                     # The try-finally block is so we can print a partial line
                     # with the name of the file, and print (changed) on the

cogapp/test_cogapp.py

@@ -824,6 +824,12 @@ def test_no_dash_o_and_amp_file(self):
         with self.assertRaisesRegex(CogUsageError, r"^Can't use -o with &file$"):
             self.cog.callable_main(["argv0", "-o", "foo", "&cogfiles.txt"])
 
+    def test_no_diff_without_check(self):
+        with self.assertRaisesRegex(
+            CogUsageError, r"^Can't use --diff without --check$"
+        ):
+            self.cog.callable_main(["argv0", "--diff"])
+
     def test_dash_v(self):
         self.assertEqual(self.cog.main(["argv0", "-v"]), 0)
         output = self.output.getvalue()
@@ -2167,6 +2173,40 @@ def test_check_bad(self):
         )
         self.assert_made_files_unchanged(d)
 
+    def test_check_bad_with_diff(self):
+        d = {
+            "skittering.cog": """\
+                //[[[cog
+                for i in range(5): cog.outl(f"number {i}")
+                cog.outl("goodbye world")
+                //]]]
+                number 0
+                number 1
+                number 2
+                number 3
+                number 4
+                hello world
+                //[[[end]]]
+                """,
+        }
+        make_files(d)
+        self.run_check(["--diff", "skittering.cog"], status=5)
+        output = """\
+            Checking skittering.cog  (changed)
+            --- current skittering.cog
+            +++ changed skittering.cog
+            @@ -7,5 +7,5 @@
+             number 2
+             number 3
+             number 4
+            -hello world
+            +goodbye world
+             //[[[end]]]
+            Check failed
+            """
+        self.assertEqual(self.output.getvalue(), reindent_block(output))
+        self.assert_made_files_unchanged(d)
+
     def test_check_mixed(self):
         d = {
             "unchanged.cog": """\

docs/running.rst

@@ -56,6 +56,7 @@ Cog is a command-line utility which takes arguments in standard form.
         -z          The end-output marker can be omitted, and is assumed at eof.
         -v          Print the version of cog and exit.
         --check     Check that the files would not change if run again.
+        --diff      With --check, show a diff of what failed the check.
         --markers='START END END-OUTPUT'
                     The patterns surrounding cog inline instructions. Should
                     include three values separated by spaces, the start, end,
@@ -65,7 +66,7 @@ Cog is a command-line utility which takes arguments in standard form.
                     1 lists only changed files, 0 lists no files.
         -h          Print this help.
 
-.. {{{end}}} (checksum: 159e7d7aebb9dcc98f250d47879703dd)
+.. {{{end}}} (checksum: d9737a8e9fc9c2fc78a5200f2fa8440f)
 
 In addition to running cog as a command on the command line, you can also
 invoke it as a module with the Python interpreter:
@@ -203,6 +204,9 @@ You can use the ``--check`` option to run cog just to check that the files
 would not change if run again.  This is useful in continuous integration to
 check that your files have been updated properly.
 
+The ``--diff`` option will show a unified diff of the change that caused
+``--check`` to fail.
+
 
 Output line suffixes
 --------------------