browser: New module for reasonably ergonomic web browser interaction

Based on `nextstrain view`'s interaction with the standard library's
webbrowser module.  Arranges to launch the browser in a separate thread,
as this will be good enough for many contexts.  `nextstrain view` itself
still arranges to launch the browser in a separate _process_, since
view's main thread/process exec's into a new program shortly after
launching the browser.

nextstrain/cli/browser.py

@@ -0,0 +1,51 @@
+"""
+Web browser interaction.
+"""
+import webbrowser
+from threading import Thread, ThreadError
+from os import environ
+from .util import warn
+
+
+# Avoid text-mode browsers
+TERM = environ.pop("TERM", None)
+try:
+    BROWSER = webbrowser.get()
+except:
+    BROWSER = None
+finally:
+    if TERM is not None:
+        environ["TERM"] = TERM
+
+
+def open_browser(url: str, new_thread: bool = True):
+    """
+    Opens *url* in a web browser.
+
+    Opens in a new tab, if possible, and raises the window to the top, if
+    possible.
+
+    Launches the browser from a separate thread by default so waiting on the
+    browser child process doesn't block the main (or calling) thread.  Set
+    *new_thread* to False to launch from the same thread as the caller (e.g. if
+    you've already spawned a dedicated thread or process for the browser).
+    Note that some registered browsers launch in the background themselves, but
+    not all do, so this feature makes launch behaviour consistent across
+    browsers.
+
+    Prints a warning to stderr if a browser can't be found or can't be
+    launched, as automatically opening a browser is considered a
+    nice-but-not-necessary feature.
+    """
+    if not BROWSER:
+        warn(f"Couldn't open <{url}> in browser: no browser found")
+        return
+
+    try:
+        if new_thread:
+            Thread(target = open_browser, args = (url, False), daemon = True).start()
+        else:
+            # new = 2 means new tab, if possible
+            BROWSER.open(url, new = 2, autoraise = True)
+    except (ThreadError, webbrowser.Error) as err:
+        warn(f"Couldn't open <{url}> in browser: {err!r}")

nextstrain/cli/command/view.py

@@ -48,7 +48,6 @@
 from multiprocessing import Process, ProcessError
 import re
 import requests
-import webbrowser
 from inspect import cleandoc
 from os import environ
 from pathlib import Path
@@ -57,6 +56,7 @@
 from typing import Iterable, NamedTuple, Tuple, Union
 from .. import runner
 from ..argparse import add_extended_help_flags, SUPPRESS, SKIP_AUTO_DEFAULT_IN_HELP
+from ..browser import BROWSER, open_browser as __open_browser
 from ..runner import docker, ambient, conda, singularity
 from ..util import colored, remove_suffix, warn
 from ..volume import NamedVolume
@@ -67,16 +67,6 @@
 PORT = environ.get("PORT") or "4000"
 
 
-# Avoid text-mode browsers
-TERM = environ.pop("TERM", None)
-try:
-    BROWSER = webbrowser.get()
-except:
-    BROWSER = None
-finally:
-    if TERM is not None:
-        environ["TERM"] = TERM
-
 OPEN_DEFAULT = bool(BROWSER)
 
 
@@ -454,8 +444,4 @@ def _open_browser(url: str):
         warn(f"Couldn't open <{url}> in browser: Auspice never started listening")
         return
 
-    try:
-        # new = 2 means new tab, if possible
-        BROWSER.open(url, new = 2, autoraise = True)
-    except webbrowser.Error as err:
-        warn(f"Couldn't open <{url}> in browser: {err!r}")
+    __open_browser(url, new_thread = False)