BUG: Improved thread safety for read_html() GH16928

doc/source/whatsnew/v0.21.0.txt

@@ -164,6 +164,8 @@ I/O
 
 - Bug in :func:`read_stata` where value labels could not be read when using an iterator (:issue:`16923`)
 
+- Bug in :func:`read_html` where import check fails when run in multiple threads (:issue:`16928`)
+
 Plotting
 ^^^^^^^^
 - Bug in plotting methods using ``secondary_y`` and ``fontsize`` not setting secondary axis font size (:issue:`12565`)

pandas/io/html.py

@@ -37,8 +37,6 @@ def _importers():
     if _IMPORTS:
         return
 
-    _IMPORTS = True
-
     global _HAS_BS4, _HAS_LXML, _HAS_HTML5LIB
 
     try:
@@ -59,6 +57,8 @@ def _importers():
     except ImportError:
         pass
 
+    _IMPORTS = True
+
 
 #############
 # READ HTML #

pandas/tests/io/test_html.py

@@ -3,13 +3,17 @@
 import glob
 import os
 import re
+import threading
 import warnings
 
+
+# imports needed for Python 3.x but will fail under Python 2.x
 try:
-    from importlib import import_module
+    from importlib import import_module, reload
 except ImportError:
     import_module = __import__
 
+
 from distutils.version import LooseVersion
 
 import pytest
@@ -22,6 +26,7 @@
 from pandas.compat import (map, zip, StringIO, string_types, BytesIO,
                            is_platform_windows, PY3)
 from pandas.io.common import URLError, urlopen, file_path_to_url
+import pandas.io.html
 from pandas.io.html import read_html
 from pandas._libs.parsers import ParserError
 
@@ -931,3 +936,32 @@ def test_same_ordering():
     dfs_lxml = read_html(filename, index_col=0, flavor=['lxml'])
     dfs_bs4 = read_html(filename, index_col=0, flavor=['bs4'])
     assert_framelist_equal(dfs_lxml, dfs_bs4)
+
+
+class ErrorThread(threading.Thread):
+    def run(self):
+        try:
+            super(ErrorThread, self).run()
+        except Exception as e:
+            self.err = e
+        else:
+            self.err = None
+
+
+@pytest.mark.slow
+def test_importcheck_thread_safety():
+    # see gh-16928
+
+    # force import check by reinitalising global vars in html.py
+    reload(pandas.io.html)
+
+    filename = os.path.join(DATA_PATH, 'valid_markup.html')
+    helper_thread1 = ErrorThread(target=read_html, args=(filename,))
+    helper_thread2 = ErrorThread(target=read_html, args=(filename,))
+
+    helper_thread1.start()
+    helper_thread2.start()
+
+    while helper_thread1.is_alive() or helper_thread2.is_alive():
+        pass
+    assert None is helper_thread1.err is helper_thread2.err