[bugfix] Use site.lang instead of site.code in http.user_agent()

- if lang is already cached in site.siteinfo, use it for values in
  http.user_agent(). Otherwise use site.code delimited in brackets
  instead.
- simplify code to update values
- update DummySiteinfo
- update documentation and type hints

Bug: T228322
Change-Id: I54403e06a7304d036c3c8587a751d050e4e3b9e6
---
M tests/utils.py
M pywikibot/comms/http.py
2 files changed, 40 insertions(+), 14 deletions(-)

diff --git a/pywikibot/comms/http.py b/pywikibot/comms/http.py
index 5ef5f3a..f682298 100644
--- a/pywikibot/comms/http.py
+++ b/pywikibot/comms/http.py
@@ -188,29 +188,26 @@
     return username
 
 
-def user_agent(site=None, format_string: str = None) -> str:
-    """
-    Generate the user agent string for a given site and format.
+def user_agent(site: Optional['pywikibot.site.BaseSite'] = None,
+               format_string: str = '') -> str:
+    """Generate the user agent string for a given site and format.
 
-    :param site: The site for which this user agent is intended. May be None.
-    :type site: BaseSite
-    :param format_string: The string to which the values will be added using
-        str.format. Is using config.user_agent_format when it is None.
+    :param site: The site for which this user agent is intended. May be
+        None.
+    :param format_string: The string to which the values will be added
+        using str.format. Is using config.user_agent_format when it is
+        empty.
     :return: The formatted user agent
     """
     values = USER_AGENT_PRODUCTS.copy()
     values.update(dict.fromkeys(['script', 'script_product'],
                                 pywikibot.bot.calledModuleName()))
+    values.update(dict.fromkeys(['family', 'code', 'lang', 'site'], ''))
 
     script_comments = []
     if config.user_agent_description:
         script_comments.append(config.user_agent_description)
 
-    values['family'] = ''
-    values['code'] = ''
-    values['lang'] = ''  # TODO: use site.lang, if known
-    values['site'] = ''
-
     username = ''
     if site:
         script_comments.append(str(site))
@@ -224,7 +221,8 @@
         values.update({
             'family': site.family.name,
             'code': site.code,
-            'lang': site.code,  # TODO: use site.lang, if known
+            'lang': (site.lang if site.siteinfo.is_cached('lang')
+                     else f'({site.code})'),
             'site': str(site),
         })
 
diff --git a/tests/utils.py b/tests/utils.py
index c2e483d..5f93270 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -244,7 +244,11 @@
 
 class DummySiteinfo:
 
-    """Dummy class to use instead of :py:obj:`pywikibot.site.Siteinfo`."""
+    """Dummy Siteinfo class.
+
+    To be used instead of :class:`pywikibot.site.Siteinfo
+    <pywikibot.site._siteinfo.Siteinfo>`.
+    """
 
     def __init__(self, cache):
         """Initializer."""
@@ -282,6 +286,13 @@
         """Return False."""
         return False
 
+    def is_cached(self, key: str) -> bool:
+        """Return whether the key is cached.
+
+        .. versionadded:: 8.3
+        """
+        return key in self._cache
+
     def is_recognised(self, key):
         """Return None."""
         return None

