pythonlib: return None from run_command() (#165)

Removing the 7.0.0 backwards compatibility from run_command()
which was raising exception on error but returning 0 on success.

Adding option to use the standard fatal() error handling
which might be good for use within GRASS GIS code itself.

Adding extensive documentation for the available options
and linking all relevant functions.

Also passing module name explicitelly to CalledModuleError
as a parameter.

lib/python/script/core.py

@@ -326,6 +326,52 @@ def make_command(prog, flags="", overwrite=False, quiet=False, verbose=False,
 
 
 def handle_errors(returncode, result, args, kwargs):
+    """Error handler for :func:`run_command()` and similar functions
+
+    The function returns *result* if *returncode* is equal to 0,
+    otherwise it reports errors based on the current settings.
+
+    The functions which are using this function to handle errors,
+    can be typically called with an *errors* parameter.
+    This function can handle one of the following values: raise,
+    fatal, status, exit, and ignore. The value raise is a default.
+
+    If *kwargs* dictionary contains key ``errors``, the value is used
+    to determine the behavior on error.
+    The value ``errors="raise"`` is a default in which case a
+    ``CalledModuleError`` exception is raised.
+
+    For ``errors="fatal"``, the function calls :func:`fatal()`
+    which has its own rules on what happens next.
+
+    For ``errors="status"``, the *returncode* will be returned.
+    This is useful, e.g., for cases when the exception-based error
+    handling mechanism is not desirable or the return code has some
+    meaning not necessarily interpreted as an error by the caller.
+
+    For ``errors="exit"``, ``sys.exit()`` is called with the
+    *returncode*, so it behaves similarly to a Bash script with
+    ``set -e``. No additional error message or exception is produced.
+    This might be useful for a simple script where error message
+    produced by the called module provides sufficient information about
+    what happened to the end user.
+
+    Finally, for ``errors="ignore"``, the value of *result* will be
+    passed in any case regardless of the *returncode*.
+    """
+    def get_module_and_code(args, kwargs):
+        """Get module name and formatted command"""
+        # TODO: construction of the whole command is far from perfect
+        args = make_command(*args, **kwargs)
+        # Since we are in error handler, let's be extra cautious
+        # about an empty command.
+        if args:
+            module = args[0]
+        else:
+            module = None
+        code = ' '.join(args)
+        return module, code
+
     if returncode == 0:
         return result
     handler = kwargs.get('errors', 'raise')
@@ -333,15 +379,19 @@ def handle_errors(returncode, result, args, kwargs):
         return result
     elif handler.lower() == 'status':
         return returncode
+    elif handler.lower() == 'fatal':
+        module, code = get_module_and_code(args, kwargs)
+        fatal(_("Module {module} ({code}) failed with"
+                " non-zero return code {returncode}").format(
+                module=module, code=code, returncode=returncode))
     elif handler.lower() == 'exit':
-        sys.exit(1)
+        sys.exit(returncode)
     else:
-        # TODO: construction of the whole command is far from perfect
-        args = make_command(*args, **kwargs)
-        code = ' '.join(args)
-        raise CalledModuleError(module=None, code=code,
+        module, code = get_module_and_code(args, kwargs)
+        raise CalledModuleError(module=module, code=code,
                                 returncode=returncode)
 
+
 def start_command(prog, flags="", overwrite=False, quiet=False,
                   verbose=False, superquiet=False, **kwargs):
     """Returns a Popen object with the command created by make_command.
@@ -398,26 +448,34 @@ def run_command(*args, **kwargs):
 
     This function passes all arguments to ``start_command()``,
     then waits for the process to complete. It is similar to
-    ``subprocess.check_call()``, but with the ``make_command()``
-    interface.
-
-    For backward compatibility, the function returns exit code
-    by default but only if it is equal to zero. An exception is raised
-    in case of an non-zero return code.
+    ``subprocess.check_call()``, but with the :func:`make_command()`
+    interface. By default, an exception is raised in case of a non-zero
+    return code by default.
 
     >>> run_command('g.region', raster='elevation')
-    0
 
     See :func:`start_command()` for details about parameters and usage.
 
-    ..note::
-        You should ignore the return value of this function unless, you
-        change the default behavior using *errors* parameter.
-
-    :param *args: unnamed arguments passed to ``start_command()``
-    :param **kwargs: named arguments passed to ``start_command()``
-
-    :returns: 0 with default parameters for backward compatibility only
+    The behavior on error can be changed using *errors* parameter
+    which is passed to the :func:`handle_errors()` function.
+
+    :param *args: unnamed arguments passed to :func:`start_command()`
+    :param **kwargs: named arguments passed to :func:`start_command()`
+    :param str errors: passed to :func:`handle_errors()`
+
+    .. versionchanged:: 8.0
+        Before 8.0, the function was returning 0 when no error occurred
+        for backward compatibility with code which was checking that
+        value. Now the function returns None, unless ``errors="status"``
+        is specified.
+    .. versionchanged:: 7.2
+        In 7.0.0, this function was returning the error code. However,
+        it was rarely checked especially outside of the core code.
+        Additionally, :func:`read_command()` needed a mechanism to
+        report errors as it was used more and more in context which
+        required error handling, Thus, exceptions were introduced as a
+        more expected default behavior for Python programmers. The
+        change was backported to 7.0 series.
 
     :raises: ``CalledModuleError`` when module returns non-zero return code
     """
@@ -438,7 +496,7 @@ def run_command(*args, **kwargs):
             sys.stderr.write(stderr)
     else:
         returncode = ps.wait()
-    return handle_errors(returncode, returncode, args, kwargs)
+    return handle_errors(returncode, result=None, args=args, kwargs=kwargs)
 
 
 def pipe_command(*args, **kwargs):
@@ -481,6 +539,9 @@ def read_command(*args, **kwargs):
     """Passes all arguments to pipe_command, then waits for the process to
     complete, returning its stdout (i.e. similar to shell `backticks`).
 
+    The behavior on error can be changed using *errors* parameter
+    which is passed to the :func:`handle_errors()` function.
+
     :param list args: list of unnamed arguments (see start_command() for details)
     :param list kwargs: list of named arguments (see start_command() for details)
 
@@ -559,6 +620,9 @@ def write_command(*args, **kwargs):
 
     See ``start_command()`` for details about parameters and usage.
 
+    The behavior on error can be changed using *errors* parameter
+    which is passed to the :func:`handle_errors()` function.
+
     :param *args: unnamed arguments passed to ``start_command()``
     :param **kwargs: named arguments passed to ``start_command()``