Minimal example to raise bad write entry error with urllib3 and certifi.

urllib3bug.py

import certifi
import urllib3.contrib.pyopenssl
from urllib3.connectionpool import connection_from_url

urllib3.contrib.pyopenssl.inject_into_urllib3()

conn = connection_from_url('https://httpbin.org/post', cert_reqs='CERT_REQUIRED', ca_certs=certifi.where())

conn.urlopen(
    method='POST',
    url='https://httpbin.org/post',
    body=u'c'*401000,
    headers={'User-Agent': 'python-requests/2.18.4',
             'Accept-Encoding': 'gzip, deflate', 'Accept': '*/*',
             'Connection': 'keep-alive', 'Content-Length': '401000'},
    redirect=False,
    assert_same_host=False,
    preload_content=False,
    decode_content=False,
    retries=1,
    timeout=30,  # timeout causes bad write entry error
)