scancode: filter license from pip pkg classifiers

When scancode detects python package licenses it attaches the license
classifiers to the declared_license dictionary, if applicable. This is a
problem when Tern tries to report the package license by adding it to a
set of licenses in get_package_licenses(), as dictionary objects cannot
be added to sets.

This commit filters out the declared license string from declared
license dictionary containing the classifier values in order to fix
this issue.

Resolves tern-tools#964

Signed-off-by: Rose Judge <rjudge@vmware.com>

tern/extensions/scancode/executor.py

@@ -75,12 +75,29 @@ def get_scancode_file(file_dict):
     return fd
 
 
+def filter_pkg_license(declared_license):
+    '''When scancode detects python package licenses, it attaches classifiers
+    to the declared_license field as a dictionary object, otherwise
+    it will represent the license as a string. Given a scancode
+    declared_license field, extract and return a license string'''
+    if isinstance(declared_license, dict):
+        try:
+            return declared_license['license']
+        except KeyError:
+            # parse classifiers for PyPI licenses
+            # According to https://pypi.org/pypi?%3Aaction=list_classifiers
+            # we can always take the value after the last '::'
+            return declared_license['classifiers'][0].split('::')[-1].strip()
+
+    return declared_license
+
+
 def get_scancode_package(package_dict):
     '''Given a package dictionary from the scancode results, return a Package
     object with the results'''
     package = Package(package_dict['name'])
     package.version = package_dict['version']
-    package.pkg_license = package_dict['declared_license']
+    package.pkg_license = filter_pkg_license(package_dict['declared_license'])
     package.copyright = package_dict['copyright']
     package.proj_url = package_dict['repository_homepage_url']
     package.download_url = package_dict['download_url']