Isolate src_prefix computation in a function

Author: xavfernandez

src/pip/_internal/cli/cmdoptions.py

@@ -18,7 +18,7 @@
 from textwrap import dedent
 
 from pip._internal.exceptions import CommandError
-from pip._internal.locations import USER_CACHE_DIR, src_prefix
+from pip._internal.locations import USER_CACHE_DIR, get_src_prefix
 from pip._internal.models.format_control import FormatControl
 from pip._internal.models.index import PyPI
 from pip._internal.models.search_scope import SearchScope
@@ -435,7 +435,7 @@ def editable():
     '--src', '--source', '--source-dir', '--source-directory',
     dest='src_dir',
     metavar='dir',
-    default=src_prefix,
+    default=get_src_prefix(),
     help='Directory to check out editable projects into. '
     'The default in a virtualenv is "<venv path>/src". '
     'The default for global installs is "<current dir>/src".'

src/pip/_internal/locations.py

@@ -23,21 +23,23 @@
 USER_CACHE_DIR = appdirs.user_cache_dir("pip")
 
 
-if running_under_virtualenv():
-    src_prefix = os.path.join(sys.prefix, 'src')
-else:
-    # FIXME: keep src in cwd for now (it is not a temporary folder)
-    try:
-        src_prefix = os.path.join(os.getcwd(), 'src')
-    except OSError:
-        # In case the current working directory has been renamed or deleted
-        sys.exit(
-            "The folder you are executing pip from can no longer be found."
-        )
+def get_src_prefix():
+    if running_under_virtualenv():
+        src_prefix = os.path.join(sys.prefix, 'src')
+    else:
+        # FIXME: keep src in cwd for now (it is not a temporary folder)
+        try:
+            src_prefix = os.path.join(os.getcwd(), 'src')
+        except OSError:
+            # In case the current working directory has been renamed or deleted
+            sys.exit(
+                "The folder you are executing pip from can no longer be found."
+            )
+
+    # under macOS + virtualenv sys.prefix is not properly resolved
+    # it is something like /path/to/python/bin/..
+    return os.path.abspath(src_prefix)
 
-# under macOS + virtualenv sys.prefix is not properly resolved
-# it is something like /path/to/python/bin/..
-src_prefix = os.path.abspath(src_prefix)
 
 # FIXME doesn't account for venv linked to global site-packages