This is a regression in Python 3.7:

    $ BROWSER=w3m python3.6 -c 'import sys; import webbrowser; print(sys.version); print(webbrowser.get()); print(webbrowser._tryorder)'
    3.6.7 (default, Nov 24 2018, 09:47:01)
    [GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.11.45.5)]
    <webbrowser.GenericBrowser object at 0x1066207f0>
    ['w3m', 'MacOSX', 'chrome', 'firefox', 'safari', 'w3m']

    $ BROWSER=w3m python3.7 -c 'import sys; import webbrowser; print(sys.version); print(webbrowser.get()); print(webbrowser._tryorder)'
    3.7.1 (default, Nov 24 2018, 09:35:18)
    [Clang 10.0.0 (clang-1000.11.45.5)]
    <webbrowser.MacOSXOSAScript object at 0x1061fe828>
    ['MacOSX', 'chrome', 'firefox', 'safari', 'w3m', 'w3m']

Note how Python 3.7.1 webbrowser decides to choose MacOSX as the default although BROWSER is explicitly set to w3m. (The above code does not run on Python 3.7.0 due to bpo-31014).

This is because of misinterpretation of the `preferred` kwarg when bpo-31014 was fixed. See https://github.com/python/cpython/commit/25b804a9c21c735ce322877f105ebab2539ccfc1#diff-546766353a40839aa72374ecca5b0925R566. Browsers in BROWSER should be registered as `preferred=True` instead.

New changeset 8c281ed403fd915284d5bba2405d7c47f8195066 by Serhiy Storchaka (Zhiming Wang) in branch 'master':
bpo-35308: Fix regression where BROWSER env var is not respected. (GH-10693)
https://github.com/python/cpython/commit/8c281ed403fd915284d5bba2405d7c47f8195066

New changeset 2a37f013ec81099a6156160ce66803b2609bb7f4 by Miss Islington (bot) in branch '3.7':
bpo-35308: Fix regression where BROWSER env var is not respected. (GH-10693)
https://github.com/python/cpython/commit/2a37f013ec81099a6156160ce66803b2609bb7f4