Make fetch_new() a class method.

src/pip/_internal/vcs/__init__.py

@@ -379,7 +379,8 @@ def compare_urls(self, url1, url2):
         """
         return (self.normalize_url(url1) == self.normalize_url(url2))
 
-    def fetch_new(self, dest, url, rev_options):
+    @classmethod
+    def fetch_new(cls, dest, url, rev_options):
         """
         Fetch a revision from a repository, in the case that this is the
         first fetch from the repository.

src/pip/_internal/vcs/bazaar.py

@@ -46,7 +46,8 @@ def export(self, location):
             show_stdout=False,
         )
 
-    def fetch_new(self, dest, url, rev_options):
+    @classmethod
+    def fetch_new(cls, dest, url, rev_options):
         rev_display = rev_options.to_display()
         logger.info(
             'Checking out %s%s to %s',
@@ -55,7 +56,7 @@ def fetch_new(self, dest, url, rev_options):
             display_path(dest),
         )
         cmd_args = ['branch', '-q'] + rev_options.to_args() + [url, dest]
-        self.run_command(cmd_args)
+        cls.run_command(cmd_args)
 
     def switch(self, dest, url, rev_options):
         self.run_command(['switch', url], cwd=dest)

src/pip/_internal/vcs/git.py

@@ -201,35 +201,36 @@ def is_commit_id_equal(cls, dest, name):
 
         return cls.get_revision(dest) == name
 
-    def fetch_new(self, dest, url, rev_options):
+    @classmethod
+    def fetch_new(cls, dest, url, rev_options):
         rev_display = rev_options.to_display()
         logger.info(
             'Cloning %s%s to %s', redact_password_from_url(url),
             rev_display, display_path(dest),
         )
-        self.run_command(['clone', '-q', url, dest])
+        cls.run_command(['clone', '-q', url, dest])
 
         if rev_options.rev:
             # Then a specific revision was requested.
-            rev_options = self.resolve_revision(dest, url, rev_options)
+            rev_options = cls.resolve_revision(dest, url, rev_options)
             branch_name = getattr(rev_options, 'branch_name', None)
             if branch_name is None:
                 # Only do a checkout if the current commit id doesn't match
                 # the requested revision.
-                if not self.is_commit_id_equal(dest, rev_options.rev):
+                if not cls.is_commit_id_equal(dest, rev_options.rev):
                     cmd_args = ['checkout', '-q'] + rev_options.to_args()
-                    self.run_command(cmd_args, cwd=dest)
-            elif self.get_current_branch(dest) != branch_name:
+                    cls.run_command(cmd_args, cwd=dest)
+            elif cls.get_current_branch(dest) != branch_name:
                 # Then a specific branch was requested, and that branch
                 # is not yet checked out.
                 track_branch = 'origin/{}'.format(branch_name)
                 cmd_args = [
                     'checkout', '-b', branch_name, '--track', track_branch,
                 ]
-                self.run_command(cmd_args, cwd=dest)
+                cls.run_command(cmd_args, cwd=dest)
 
         #: repo may contain submodules
-        self.update_submodules(dest)
+        cls.update_submodules(dest)
 
     def switch(self, dest, url, rev_options):
         self.run_command(['config', 'remote.origin.url', url], cwd=dest)

src/pip/_internal/vcs/mercurial.py

@@ -32,17 +32,18 @@ def export(self, location):
                 ['archive', location], show_stdout=False, cwd=temp_dir.path
             )
 
-    def fetch_new(self, dest, url, rev_options):
+    @classmethod
+    def fetch_new(cls, dest, url, rev_options):
         rev_display = rev_options.to_display()
         logger.info(
             'Cloning hg %s%s to %s',
             url,
             rev_display,
             display_path(dest),
         )
-        self.run_command(['clone', '--noupdate', '-q', url, dest])
+        cls.run_command(['clone', '--noupdate', '-q', url, dest])
         cmd_args = ['update', '-q'] + rev_options.to_args()
-        self.run_command(cmd_args, cwd=dest)
+        cls.run_command(cmd_args, cwd=dest)
 
     def switch(self, dest, url, rev_options):
         repo_config = os.path.join(dest, self.dirname, 'hgrc')

src/pip/_internal/vcs/subversion.py

@@ -46,7 +46,8 @@ def export(self, location):
             cmd_args = ['export'] + rev_options.to_args() + [url, location]
             self.run_command(cmd_args, show_stdout=False)
 
-    def fetch_new(self, dest, url, rev_options):
+    @classmethod
+    def fetch_new(cls, dest, url, rev_options):
         rev_display = rev_options.to_display()
         logger.info(
             'Checking out %s%s to %s',
@@ -55,7 +56,7 @@ def fetch_new(self, dest, url, rev_options):
             display_path(dest),
         )
         cmd_args = ['checkout', '-q'] + rev_options.to_args() + [url, dest]
-        self.run_command(cmd_args)
+        cls.run_command(cmd_args)
 
     def switch(self, dest, url, rev_options):
         cmd_args = ['switch'] + rev_options.to_args() + [url, dest]