ERR: include original error message for missing required dependencies (#26665)

Author: DanielFEvans

doc/source/whatsnew/v0.25.0.rst

@@ -82,7 +82,7 @@ Other Enhancements
 - :meth:`DataFrame.query` and :meth:`DataFrame.eval` now supports quoting column names with backticks to refer to names with spaces (:issue:`6508`)
 - :func:`merge_asof` now gives a more clear error message when merge keys are categoricals that are not equal (:issue:`26136`)
 - :meth:`pandas.core.window.Rolling` supports exponential (or Poisson) window type (:issue:`21303`)
--
+- Error message for missing required imports now includes the original ImportError's text (:issue:`23868`)
 
 .. _whatsnew_0250.api_breaking:
 

pandas/__init__.py

@@ -10,11 +10,13 @@
     try:
         __import__(dependency)
     except ImportError as e:
-        missing_dependencies.append(dependency)
+        missing_dependencies.append((dependency, e))
 
 if missing_dependencies:
-    raise ImportError(
-        "Missing required dependencies {0}".format(missing_dependencies))
+    msg = "Unable to import required dependencies:"
+    for dependency, e in missing_dependencies:
+        msg += "\n{0}: {1}".format(dependency, str(e))
+    raise ImportError(msg)
 del hard_dependencies, dependency, missing_dependencies
 
 # numpy compat

pandas/tests/test_base.py

@@ -1,7 +1,9 @@
 from datetime import datetime, timedelta
+from importlib import reload
 from io import StringIO
 import re
 import sys
+from unittest.mock import patch
 
 import numpy as np
 import pytest
@@ -1341,3 +1343,28 @@ def test_to_numpy_dtype(as_series):
     expected = np.array(['2000-01-01T05', '2001-01-01T05'],
                         dtype='M8[ns]')
     tm.assert_numpy_array_equal(result, expected)
+
+
+@patch("builtins.__import__")
+def test_missing_required_dependency(mock_import):
+    def mock_import_fail(name, *args, **kwargs):
+        if name == "numpy":
+            raise ImportError("cannot import name numpy")
+        elif name == "pytz":
+            raise ImportError("cannot import name some_dependency")
+        elif name == "dateutil":
+            raise ImportError("cannot import name some_other_dependency")
+        else:
+            return __import__(name, *args, **kwargs)
+
+    mock_import.side_effect = mock_import_fail
+
+    expected_msg = (
+        "Unable to import required dependencies:"
+        "\nnumpy: cannot import name numpy"
+        "\npytz: cannot import name some_dependency"
+        "\ndateutil: cannot import name some_other_dependency"
+    )
+
+    with pytest.raises(ImportError, match=expected_msg):
+        reload(pd)