Add new build_spdx_license_expression() function

This new function returns an SPDX license expression string from a
ScanCode license_expression string.

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

src/licensedcode/cache.py

@@ -363,3 +363,19 @@ def get_spdx_symbols(licenses_db=None, check_consistency=SCANCODE_DEV_MODE):
     if licenses_db:
         return build_spdx_symbols(licenses_db)
     return get_cache(check_consistency=check_consistency).spdx_symbols
+
+
+def build_spdx_license_expression(license_expression, licensing=None):
+    """
+    Return an SPDX license expression from a ScanCode ``license_expression``
+    string.
+
+    For example::
+    >>> exp = "mit OR gpl-2.0 with generic-exception"
+    >>> spdx = "MIT OR GPL-2.0-only WITH LicenseRef-scancode-generic-exception"
+    >>> assert build_spdx_license_expression(exp) == spdx
+    """
+    if not licensing:
+        licensing = get_licensing()
+    parsed = licensing.parse(license_expression)
+    return parsed.render(template='{symbol.wrapped.spdx_license_key}')

src/licensedcode/models.py

@@ -991,11 +991,8 @@ def licensing_contains(self, other):
             )
 
     def spdx_license_expression(self, licensing=None):
-        if not licensing:
-            from licensedcode.cache import get_licensing
-            licensing = get_licensing()
-        parsed = licensing.parse(self.license_expression)
-        return parsed.render(template='{symbol.spdx_license_key}')
+        from licensedcode.cache import build_spdx_license_expression
+        return build_spdx_license_expression(self, licensing=licensing)
 
     def get_length(self, unique=False):
         return self.length_unique if unique else self.length