Fixed pandas-dev#16583 by coercing mode to 'r'

Adjusted tests on warnings/errors to account for the change

pandas/io/pytables.py

@@ -313,10 +313,10 @@ def read_hdf(path_or_buf, key=None, **kwargs):
 
         """
 
-    if kwargs.get('mode', 'a') not in ['r', 'r+', 'a']:
-        raise ValueError('mode {0} is not allowed while performing a read. '
-                         'Allowed modes are r, r+ and a.'
-                         .format(kwargs.get('mode')))
+    # Ignore any supplied mode, we only need to read data
+    if kwargs.pop('mode', 'r') != 'r':
+        warnings.warn('Ignoring requested mode to read_hdf, opening read-only')
+
     # grab the scope
     if 'where' in kwargs:
         kwargs['where'] = _ensure_term(kwargs['where'], scope_level=1)
@@ -343,7 +343,7 @@ def read_hdf(path_or_buf, key=None, **kwargs):
             raise compat.FileNotFoundError(
                 'File %s does not exist' % path_or_buf)
 
-        store = HDFStore(path_or_buf, **kwargs)
+        store = HDFStore(path_or_buf, mode='r', **kwargs)
         # can't auto open/close if we are using an iterator
         # so delegate to the iterator
         auto_close = True

pandas/tests/io/test_pytables.py

@@ -512,12 +512,12 @@ def f():
                     df.to_hdf(path, 'df', mode=mode)
 
                 # conv read
-                if mode in ['w']:
-                    pytest.raises(ValueError, read_hdf,
-                                  path, 'df', mode=mode)
+                if mode != 'r':
+                    with pytest.warns(UserWarning):
+                        result = read_hdf(path, 'df', mode=mode)
                 else:
                     result = read_hdf(path, 'df', mode=mode)
-                    assert_frame_equal(result, df)
+                assert_frame_equal(result, df)
 
         def check_default_mode():