bpo-24241: xdg-settings for preferred X browser

The traditional first entry in the tryorder queue is xdg-open, which
doesn't support new window. Make the default browser first
in the try list, to properly support these options.

Author: davesteele

Lib/webbrowser.py

@@ -13,13 +13,18 @@
 class Error(Exception):
     pass
 
-_browsers = {}          # Dictionary of available browser controllers
-_tryorder = []          # Preference order of available browsers
+_browsers = {}                  # Dictionary of available browser controllers
+_tryorder = []                  # Preference order of available browsers
+_os_preferred_browser = None    # The preferred browser
 
 def register(name, klass, instance=None, *, preferred=False):
     """Register a browser connector."""
     _browsers[name.lower()] = [klass, instance]
-    if preferred:
+
+    # Preferred browsers go to the front of the list.
+    # Need to match to the default browser returned by xdg-settings, which
+    # may be of the form e.g. "firefox.desktop".
+    if preferred or (_os_preferred_browser and name in _os_preferred_browser):
         _tryorder.insert(0, name)
     else:
         _tryorder.append(name)
@@ -484,6 +489,14 @@ def register_X_browsers():
 
 # Prefer X browsers if present
 if os.environ.get("DISPLAY"):
+    try:
+        cmd = "xdg-settings get default-web-browser".split()
+        result = subprocess.check_output(cmd).decode().strip()
+    except (FileNotFoundError, subprocess.CalledProcessError):
+        pass
+    else:
+        _os_preferred_browser = result
+
     register_X_browsers()
 
 # Also try console browsers

Misc/ACKS

@@ -1458,6 +1458,7 @@ Quentin Stafford-Fraser
 Frank Stajano
 Joel Stanley
 Anthony Starks
+David Steele
 Oliver Steele
 Greg Stein
 Marek Stepniowski

Misc/NEWS

@@ -433,6 +433,11 @@ Library
 - Issue #23262: The webbrowser module now supports Firefox 36+ and derived
   browsers.  Based on patch by Oleg Broytman.
 
+- Issue #24241: The webbrowser in an X environment now prefers using the
+  default browser directly. Also, the webbrowser register() function now has
+  a documented 'preferred' argument, to specify browsers to be returned by
+  get() with no arguments. Patch by David Steele
+
 - Issue #27939: Fixed bugs in tkinter.ttk.LabeledScale and tkinter.Scale caused
   by representing the scale as float value internally in Tk.  tkinter.IntVar
   now works if float value is set to underlying Tk variable.