Add support for chained exceptions (#63)

better_exceptions/formatter.py

@@ -276,13 +276,39 @@ def format_traceback(self, tb=None):
 
         return ''.join(lines), final_source
 
-    def format_exception(self, exc, value, tb):
-        formatted, colored_source = self.format_traceback(tb)
+    def _format_exception(self, value, tb, seen=None):
+        # Implemented from built-in traceback module:
+        # https://github.com/python/cpython/blob/a5b76167dedf4d15211a216c3ca7b98e3cec33b8/Lib/traceback.py#L468
+
+        exc_type, exc_value, exc_traceback = type(value), value, tb
+
+        if seen is None:
+            seen = set()
+
+        seen.add(id(exc_value))
+
+        if exc_value and PY3:
+            if exc_value.__cause__ is not None and id(exc_value.__cause__) not in seen:
+                for text in self._format_exception(exc_value.__cause__,exc_value.__cause__.__traceback__, seen=seen):
+                    yield text
+                yield u"\nThe above exception was the direct cause of the following exception:\n\n"
+            elif exc_value.__context__ is not None and id(exc_value.__context__) not in seen and not exc_value.__suppress_context__:
+                for text in self._format_exception(exc_value.__context__, exc_value.__context__.__traceback__, seen=seen):
+                    yield text
+                yield u"\nDuring handling of the above exception, another exception occurred:\n\n"
 
-        if not str(value) and exc is AssertionError:
+        if exc_traceback is not None:
+            yield u'Traceback (most recent call last):\n'
+
+        formatted, colored_source = self.format_traceback(exc_traceback)
+
+        yield formatted
+
+        if not str(value) and exc_type is AssertionError:
             value.args = (colored_source,)
-        title = traceback.format_exception_only(exc, value)
+        title = traceback.format_exception_only(exc_type, value)
 
-        full_trace = u'Traceback (most recent call last):\n{}{}\n'.format(formatted, ''.join(title).strip())
+        yield u''.join(title).strip() + u'\n'
 
-        return full_trace
+    def format_exception(self, exc, value, tb):
+        return u''.join(formatted for formatted in self._format_exception(value, tb))

test/output/python3-dumb-UTF-8-color.out

@@ -209,3 +209,42 @@ SyntaxError: invalid syntax
 
 
 
+python3 test/test_chaining.py
+
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 11, in cause
+    div(x, y)
+    │   │  └ 0
+    │   └ 1
+    └ <function div at 0xDEADBEEF>
+  File "test/test_chaining.py", line 7, in div
+    x / y
+    │   └ 0
+    └ 1
+ZeroDivisionError: division by zero
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 17, in context
+    cause(x, y)
+    │     │  └ 0
+    │     └ 1
+    └ <function cause at 0xDEADBEEF>
+  File "test/test_chaining.py", line 13, in cause
+    raise ValueError('Division error')
+ValueError: Division error
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 21, in <module>
+    context(1, 0)
+    └ <function context at 0xDEADBEEF>
+  File "test/test_chaining.py", line 19, in context
+    raise ValueError('Cause error') from e
+ValueError: Cause error
+
+
+

test/output/python3-dumb-UTF-8-nocolor.out

@@ -209,3 +209,42 @@ SyntaxError: invalid syntax
 
 
 
+python3 test/test_chaining.py
+
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 11, in cause
+    div(x, y)
+    │   │  └ 0
+    │   └ 1
+    └ <function div at 0xDEADBEEF>
+  File "test/test_chaining.py", line 7, in div
+    x / y
+    │   └ 0
+    └ 1
+ZeroDivisionError: division by zero
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 17, in context
+    cause(x, y)
+    │     │  └ 0
+    │     └ 1
+    └ <function cause at 0xDEADBEEF>
+  File "test/test_chaining.py", line 13, in cause
+    raise ValueError("Division error")
+ValueError: Division error
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 21, in <module>
+    context(1, 0)
+    └ <function context at 0xDEADBEEF>
+  File "test/test_chaining.py", line 19, in context
+    raise ValueError("Cause error") from e
+ValueError: Cause error
+
+
+

test/output/python3-dumb-ascii-color.out

@@ -209,3 +209,42 @@ SyntaxError: invalid syntax
 
 
 
+python3 test/test_chaining.py
+
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 11, in cause
+    div(x, y)
+    |   |  -> 0
+    |   -> 1
+    -> <function div at 0xDEADBEEF>
+  File "test/test_chaining.py", line 7, in div
+    x / y
+    |   -> 0
+    -> 1
+ZeroDivisionError: division by zero
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 17, in context
+    cause(x, y)
+    |     |  -> 0
+    |     -> 1
+    -> <function cause at 0xDEADBEEF>
+  File "test/test_chaining.py", line 13, in cause
+    raise ValueError('Division error')
+ValueError: Division error
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 21, in <module>
+    context(1, 0)
+    -> <function context at 0xDEADBEEF>
+  File "test/test_chaining.py", line 19, in context
+    raise ValueError('Cause error') from e
+ValueError: Cause error
+
+
+

test/output/python3-dumb-ascii-nocolor.out

@@ -209,3 +209,42 @@ SyntaxError: invalid syntax
 
 
 
+python3 test/test_chaining.py
+
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 11, in cause
+    div(x, y)
+    |   |  -> 0
+    |   -> 1
+    -> <function div at 0xDEADBEEF>
+  File "test/test_chaining.py", line 7, in div
+    x / y
+    |   -> 0
+    -> 1
+ZeroDivisionError: division by zero
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 17, in context
+    cause(x, y)
+    |     |  -> 0
+    |     -> 1
+    -> <function cause at 0xDEADBEEF>
+  File "test/test_chaining.py", line 13, in cause
+    raise ValueError("Division error")
+ValueError: Division error
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 21, in <module>
+    context(1, 0)
+    -> <function context at 0xDEADBEEF>
+  File "test/test_chaining.py", line 19, in context
+    raise ValueError("Cause error") from e
+ValueError: Cause error
+
+
+

test/output/python3-vt100-UTF-8-color.out

@@ -209,3 +209,42 @@ SyntaxError: invalid syntax
 
 
 
+python3 test/test_chaining.py
+
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 11, in cause
+    div(x, y)
+    │   │  └ 0
+    │   └ 1
+    └ <function div at 0xDEADBEEF>
+  File "test/test_chaining.py", line 7, in div
+    x / y
+    │   └ 0
+    └ 1
+ZeroDivisionError: division by zero
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 17, in context
+    cause(x, y)
+    │     │  └ 0
+    │     └ 1
+    └ <function cause at 0xDEADBEEF>
+  File "test/test_chaining.py", line 13, in cause
+    raise ValueError('Division error')
+ValueError: Division error
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 21, in <module>
+    context(1, 0)
+    └ <function context at 0xDEADBEEF>
+  File "test/test_chaining.py", line 19, in context
+    raise ValueError('Cause error') from e
+ValueError: Cause error
+
+
+

test/output/python3-vt100-UTF-8-nocolor.out

@@ -209,3 +209,42 @@ SyntaxError: invalid syntax
 
 
 
+python3 test/test_chaining.py
+
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 11, in cause
+    div(x, y)
+    │   │  └ 0
+    │   └ 1
+    └ <function div at 0xDEADBEEF>
+  File "test/test_chaining.py", line 7, in div
+    x / y
+    │   └ 0
+    └ 1
+ZeroDivisionError: division by zero
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 17, in context
+    cause(x, y)
+    │     │  └ 0
+    │     └ 1
+    └ <function cause at 0xDEADBEEF>
+  File "test/test_chaining.py", line 13, in cause
+    raise ValueError("Division error")
+ValueError: Division error
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 21, in <module>
+    context(1, 0)
+    └ <function context at 0xDEADBEEF>
+  File "test/test_chaining.py", line 19, in context
+    raise ValueError("Cause error") from e
+ValueError: Cause error
+
+
+

test/output/python3-vt100-ascii-color.out

@@ -209,3 +209,42 @@ SyntaxError: invalid syntax
 
 
 
+python3 test/test_chaining.py
+
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 11, in cause
+    div(x, y)
+    |   |  -> 0
+    |   -> 1
+    -> <function div at 0xDEADBEEF>
+  File "test/test_chaining.py", line 7, in div
+    x / y
+    |   -> 0
+    -> 1
+ZeroDivisionError: division by zero
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 17, in context
+    cause(x, y)
+    |     |  -> 0
+    |     -> 1
+    -> <function cause at 0xDEADBEEF>
+  File "test/test_chaining.py", line 13, in cause
+    raise ValueError('Division error')
+ValueError: Division error
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 21, in <module>
+    context(1, 0)
+    -> <function context at 0xDEADBEEF>
+  File "test/test_chaining.py", line 19, in context
+    raise ValueError('Cause error') from e
+ValueError: Cause error
+
+
+

test/output/python3-vt100-ascii-nocolor.out

@@ -209,3 +209,42 @@ SyntaxError: invalid syntax
 
 
 
+python3 test/test_chaining.py
+
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 11, in cause
+    div(x, y)
+    |   |  -> 0
+    |   -> 1
+    -> <function div at 0xDEADBEEF>
+  File "test/test_chaining.py", line 7, in div
+    x / y
+    |   -> 0
+    -> 1
+ZeroDivisionError: division by zero
+
+During handling of the above exception, another exception occurred:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 17, in context
+    cause(x, y)
+    |     |  -> 0
+    |     -> 1
+    -> <function cause at 0xDEADBEEF>
+  File "test/test_chaining.py", line 13, in cause
+    raise ValueError("Division error")
+ValueError: Division error
+
+The above exception was the direct cause of the following exception:
+
+Traceback (most recent call last):
+  File "test/test_chaining.py", line 21, in <module>
+    context(1, 0)
+    -> <function context at 0xDEADBEEF>
+  File "test/test_chaining.py", line 19, in context
+    raise ValueError("Cause error") from e
+ValueError: Cause error
+
+
+