fix(dry-run): show diff with dry-run option

README.rst

@@ -124,21 +124,22 @@ this as a fallback in the case a suitable wheel cannot be found.
 Dry run mode
 ============
 
-There are some use cases, when you maybe don't want to edit your requirements.txt
-right away. You can use the ``-dry-run`` argument to just print the lines, as if they
-would be added to your requirements.txt file.
+There are some use cases, when you maybe don't want to edit your ``requirements.txt``
+right away. You can use the ``--dry-run`` argument to show the diff, so you
+can preview the changes to your ``requirements.txt`` file.
 
 Example::
 
     hashin --dry-run requests==2.19.1
 
 Would result in a printout on the command line::
 
-    requests==2.19.1 \
-        --hash=sha256:63b52e3c866428a224f97cab011de738c36aec0185aa91cfacd418b5d58911d1 \
-        --hash=sha256:ec22d826a36ed72a7358ff3fe56cbd4ba69dd7a6718ffd450ff0e9df7a47ce6a
-
-
+    --- Old
+    +++ New
+    @@ -0,0 +1,3 @@
+    +requests==2.19.1 \
+    +    --hash=sha256:63b52e3c866428a224f97cab011de738c36aec0185aa91cfacd418b5d58911d1 \
+    +    --hash=sha256:ec22d826a36ed72a7358ff3fe56cbd4ba69dd7a6718ffd450ff0e9df7a47ce6a
 
 PEP-0496 Environment Markers
 ============================

hashin.py

@@ -15,6 +15,7 @@
 
 import pip_api
 from packaging.version import parse
+import difflib
 
 if sys.version_info >= (3,):
     from urllib.request import urlopen
@@ -77,7 +78,7 @@
 )
 parser.add_argument(
     '--dry-run',
-    help='Don\'t touch requirements.txt and just show hashes',
+    help="Don't touch requirements.txt and just show the diff",
     action='store_true',
     default=False,
 )
@@ -181,22 +182,31 @@ def run_single_package(
             new_lines += ' \\'
         new_lines += '\n'
 
-    if dry_run:
-        if verbose:
-            _verbose('Dry run, not editing ', file)
-        print(new_lines)
-        return
-    if verbose:
-        _verbose('Editing', file)
     with open(file) as f:
-        requirements = f.read()
+        old_requirements = f.read()
     requirements = amend_requirements_content(
-        requirements,
+        old_requirements,
         package,
         new_lines
     )
-    with open(file, 'w') as f:
-        f.write(requirements)
+    if dry_run:
+        if verbose:
+            _verbose('Dry run, not editing ', file)
+        print(
+            "".join(
+                difflib.unified_diff(
+                    old_requirements.splitlines(True),
+                    requirements.splitlines(True),
+                    fromfile="Old",
+                    tofile="New",
+                )
+            )
+        )
+    else:
+        with open(file, 'w') as f:
+            f.write(requirements)
+        if verbose:
+            _verbose('Editing', file)
 
 
 def amend_requirements_content(requirements, package, new_lines):

tests/test_cli.py

@@ -745,10 +745,10 @@ def mocked_get(url, **options):
         # Check dry run output
         out_lines = my_stdout.getvalue().splitlines()
         self.assertTrue(
-            'hashin==0.10' in out_lines[0]
+            '+hashin==0.10' in out_lines[3]
         )
         self.assertTrue(
-            '--hash=sha256:aaaaa' in out_lines[1]
+            '+--hash=sha256:aaaaa' in out_lines[4].replace(" ", "")
         )
 
     @cleanup_tmpdir('hashin*')