Simplify Exit Exception

= Refactor BaseCommand to simplify Exit handling

BaseCommand.main() will get the return value from `self.invoke` and
pass it to `ctx.exit()`, so that commands which `return 1` will
`ctx.exit(1)`
This is not a behavioral change -- `ctx.exit()` already does this.

Do not reraise Exit exceptions when invoked non-standalone. Instead,
return their exit codes as the result of `BaseCommand.main`. So, a
command which explicitly invokes `ctx.exit(0)` in its execution will
effectively `return 0` instead of raising a specialized exception.

= Make Exit its own RuntimeError subtype, don't pollute stdio on exit

Exit exceptions should not be a subtype of ClickException with the
output pretty-printing (`show()`) which happens when they are raised in
non-standalone mode. That would make `ctx.exit(...)` needlessly pollute
stderr. This would have downstream impact on everyone using context exit
calls, and generally be a Bad Thing (tm).

Instead, like Abort, Exit is a subclass of RuntimeError with very little
"meat on its bones". It's an exception containing a single integer (exit
code)  which is then interpreted in that way in BaseCommand.main

closes pallets#533
closes pallets#667

click/core.py

@@ -498,7 +498,7 @@ def abort(self):
 
     def exit(self, code=0):
         """Exits the application with a given exit code."""
-        raise Exit(code, self)
+        raise Exit(code)
 
     def get_usage(self):
         """Helper method to get formatted usage string for the current
@@ -714,16 +714,10 @@ def main(self, args=None, prog_name=None, complete_var=None,
                     rv = self.invoke(ctx)
                     if not standalone_mode:
                         return rv
-                    ctx.exit()
+                    ctx.exit(rv)
             except (EOFError, KeyboardInterrupt):
                 echo(file=sys.stderr)
                 raise Abort()
-            except Exit as e:
-                if not standalone_mode:
-                    if e.exit_code:
-                        raise
-                    return None
-                sys.exit(e.exit_code)
             except ClickException as e:
                 if not standalone_mode:
                     raise
@@ -734,6 +728,11 @@ def main(self, args=None, prog_name=None, complete_var=None,
                     sys.exit(1)
                 else:
                     raise
+        except Exit as e:
+            if standalone_mode:
+                sys.exit(e.exit_code)
+            else:
+                return e.exit_code
         except Abort:
             if not standalone_mode:
                 raise

click/exceptions.py

@@ -13,7 +13,6 @@ class ClickException(Exception):
 
     #: The exit code for this exception
     exit_code = 1
-    show_prefix = 'Error: '
 
     def __init__(self, message):
         ctor_msg = message
@@ -38,17 +37,19 @@ def __str__(self):
     def show(self, file=None):
         if file is None:
             file = get_text_stderr()
-        echo('%s%s' % (self.show_prefix, self.format_message()), file=file)
+        echo('Error: %s' % self.format_message(), file=file)
 
 
-class ContextAwareException(ClickException):
-    """An exception that knows how to use a :class:`~click.Context` object
-    to properly display itself.
+class UsageError(ClickException):
+    """An internal exception that signals a usage error.  This typically
+    aborts any further handling.
 
     :param message: the error message to display.
     :param ctx: optionally the context that caused this error.  Click will
                 fill in the context automatically in some situations.
     """
+    exit_code = 2
+
     def __init__(self, message, ctx=None):
         ClickException.__init__(self, message)
         self.ctx = ctx
@@ -66,33 +67,7 @@ def show(self, file=None):
         if self.ctx is not None:
             color = self.ctx.color
             echo(self.ctx.get_usage() + '\n%s' % hint, file=file, color=color)
-        echo('%s%s' % (self.show_prefix, self.format_message()), file=file, color=color)
-
-
-class Exit(ContextAwareException):
-    """An exception that indicates that the application should exit with some
-    status code.
-
-    :param code: the status code to exit with.
-    :param ctx: optionally the context that caused this error.  Click will
-                fill in the context automatically in some situations.
-    """
-    show_prefix = 'Exit: '
-
-    def __init__(self, code, ctx=None):
-        ContextAwareException.__init__(self, str(code), ctx=None)
-        self.exit_code = code
-
-
-class UsageError(ContextAwareException):
-    """An internal exception that signals a usage error.  This typically
-    aborts any further handling.
-
-    :param message: the error message to display.
-    :param ctx: optionally the context that caused this error.  Click will
-                fill in the context automatically in some situations.
-    """
-    exit_code = 2
+        echo('Error: %s' % self.format_message(), file=file, color=color)
 
 
 class BadParameter(UsageError):
@@ -248,3 +223,13 @@ def format_message(self):
 
 class Abort(RuntimeError):
     """An internal signalling exception that signals Click to abort."""
+
+
+class Exit(RuntimeError):
+    """An exception that indicates that the application should exit with some
+    status code.
+
+    :param code: the status code to exit with.
+    """
+    def __init__(self, code):
+        self.exit_code = code