feat: add dry-run option

README.rst

@@ -121,6 +121,26 @@ these exact identifiers directly, if you need something specific.
 The ``source`` release is always automatically included. ``pip`` will use
 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 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::
+
+    --- 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
@@ -75,6 +76,12 @@
     action='store_true',
     default=False,
 )
+parser.add_argument(
+    '--dry-run',
+    help="Don't touch requirements.txt and just show the diff",
+    action='store_true',
+    default=False,
+)
 
 
 major_pip_version = int(pip_api.version().split('.')[0])
@@ -101,7 +108,7 @@ def _download(url, binary=False):
     # Note that urlopen will, by default, follow redirects.
     status_code = r.getcode()
 
-    if status_code >= 301 and status_code < 400:
+    if 301 <= status_code < 400:
         location, _ = cgi.parse_header(r.headers.get('location', ''))
         if not location:
             raise PackageError("No 'Location' header on {0} ({1})".format(
@@ -137,6 +144,7 @@ def run_single_package(
     python_versions=None,
     verbose=False,
     include_prereleases=False,
+    dry_run=False,
 ):
     restriction = None
     if ';' in spec:
@@ -159,7 +167,6 @@ def run_single_package(
     package = data['package']
 
     maybe_restriction = '' if not restriction else '; {0}'.format(restriction)
-    new_lines = ''
     new_lines = '{0}=={1}{2} \\\n'.format(
         package,
         data['version'],
@@ -175,17 +182,31 @@ def run_single_package(
             new_lines += ' \\'
         new_lines += '\n'
 
-    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):
@@ -498,6 +519,7 @@ def main():
             args.python_version,
             verbose=args.verbose,
             include_prereleases=args.include_prereleases,
+            dry_run=args.dry_run,
         )
     except PackageError as exception:
         print(str(exception), file=sys.stderr)

tests/test_arg_parse.py

@@ -9,7 +9,7 @@ def test_everything():
         '-r', 'reqs.txt',
         '-a', 'sha512',
         '-p', '3.5',
-        '-v',
+        '-v', '--dry-run'
     ])
     expected = argparse.Namespace(
         algorithm='sha512',
@@ -19,6 +19,7 @@ def test_everything():
         verbose=True,
         version=False,
         include_prereleases=False,
+        dry_run=True,
     )
     assert args == (expected, [])
 
@@ -30,6 +31,7 @@ def test_everything_long():
         '--algorithm', 'sha512',
         '--python-version', '3.5',
         '--verbose',
+        '--dry-run',
     ])
     expected = argparse.Namespace(
         algorithm='sha512',
@@ -39,6 +41,7 @@ def test_everything_long():
         verbose=True,
         version=False,
         include_prereleases=False,
+        dry_run=True,
     )
     assert args == (expected, [])
 
@@ -53,5 +56,6 @@ def test_minimal():
         verbose=False,
         version=False,
         include_prereleases=False,
+        dry_run=False,
     )
     assert args == (expected, [])

tests/test_cli.py

@@ -685,6 +685,72 @@ def mocked_get(url, **options):
                 'hashin==0.11 \\'
             )
 
+    @cleanup_tmpdir('hashin*')
+    @mock.patch('hashin.urlopen')
+    def test_run_dry(self, murlopen):
+        """dry run should edit the requirements.txt file and print
+        hashes and package name in the console
+        """
+
+        def mocked_get(url, **options):
+            if url == 'https://pypi.org/pypi/hashin/json':
+                return _Response({
+                    'info': {
+                        'version': '0.11',
+                        'name': 'hashin',
+                    },
+                    'releases': {
+                        '0.11': [
+                            {
+                                'url': 'https://pypi.org/packages/source/p/hashin/hashin-0.11.tar.gz',
+                                'digests': {
+                                    'sha256': 'bbbbb',
+                                },
+                            }
+                        ],
+                        '0.10': [
+                            {
+                                'url': 'https://pypi.org/packages/source/p/hashin/hashin-0.10.tar.gz',
+                                'digests': {
+                                    'sha256': 'aaaaa',
+                                },
+                            }
+                        ]
+                    }
+                })
+
+        murlopen.side_effect = mocked_get
+
+        with tmpfile() as filename:
+            with open(filename, 'w') as f:
+                f.write('')
+
+            my_stdout = StringIO()
+            with redirect_stdout(my_stdout):
+                retcode = hashin.run(
+                    'hashin==0.10',
+                    filename,
+                    'sha256',
+                    verbose=False,
+                    dry_run=True,
+                )
+
+                self.assertEqual(retcode, 0)
+
+            # verify that nothing has been written to file
+            with open(filename) as f:
+                output = f.read()
+            assert not output
+
+        # Check dry run output
+        out_lines = my_stdout.getvalue().splitlines()
+        self.assertTrue(
+            '+hashin==0.10' in out_lines[3]
+        )
+        self.assertTrue(
+            '+--hash=sha256:aaaaa' in out_lines[4].replace(" ", "")
+        )
+
     @cleanup_tmpdir('hashin*')
     @mock.patch('hashin.urlopen')
     def test_run_pep_0496(self, murlopen):