add context manager for failed tests

Author: hamelsmu

fastcore/_nbdev.py

@@ -17,6 +17,7 @@
          "TEST_IMAGE": "00_test.ipynb",
          "TEST_IMAGE_BW": "00_test.ipynb",
          "test_fig_exists": "00_test.ipynb",
+         "TestFail": "00_test.ipynb",
          "defaults": "01_foundation.ipynb",
          "copy_func": "01_foundation.ipynb",
          "patch_to": "01_foundation.ipynb",

fastcore/test.py

@@ -1,10 +1,11 @@
 # AUTOGENERATED! DO NOT EDIT! File to edit: nbs/00_test.ipynb (unless otherwise specified).
 
 __all__ = ['test_fail', 'test', 'nequals', 'test_eq', 'test_eq_type', 'test_ne', 'is_close', 'test_close', 'test_is',
-           'test_shuffled', 'test_stdout', 'test_warns', 'TEST_IMAGE', 'TEST_IMAGE_BW', 'test_fig_exists']
+           'test_shuffled', 'test_stdout', 'test_warns', 'TEST_IMAGE', 'TEST_IMAGE_BW', 'test_fig_exists', 'TestFail']
 
 # Cell
 from .imports import *
+from .utils import *
 from collections import Counter
 from contextlib import redirect_stdout
 
@@ -95,4 +96,16 @@ def test_warns(f, show=False):
 # Cell
 def test_fig_exists(ax):
     "Test there is a figure displayed in `ax`"
-    assert ax and len(ax.figure.canvas.tostring_argb())
+    assert ax and len(ax.figure.canvas.tostring_argb())
+
+# Cell
+class TestFail():
+    "A context manager to allow you to test if an exception (e) is raised. Optionally, search the exception's error message with regex."
+    def __init__(self, ex:Exception, regex:str=None):store_attr()
+    def __enter__(self): pass
+    def __exit__(self, type, value, traceback):
+        if isinstance(value, self.ex):
+            if self.regex:
+                assert re.search(self.regex, ' '.join(value.args)) is not None, f"Did not find regex:{self.regex} in Exception message."
+            return True
+        else: raise TypeError(f"An error of {self.ex} was not raised.")