[core] I/O formats allowing both arg0 formating and kwargs based. Starting with 0.4, kwargs based will be default (BC break here, but needed for the greater good).

bonobo/settings.py

@@ -2,6 +2,8 @@ import os
 
 import logging
 
+from bonobo.errors import ValidationError
+
 
 def to_bool(s):
     if len(s):
@@ -23,7 +25,30 @@ QUIET = to_bool(os.environ.get('QUIET', 'f'))
 # Logging level.
 LOGGING_LEVEL = logging.DEBUG if DEBUG else logging.INFO
 
+# Input/Output format for transformations
+IOFORMAT_ARG0 = 'arg0'
+IOFORMAT_KWARGS = 'kwargs'
+
+IOFORMATS = {
+    IOFORMAT_ARG0,
+    IOFORMAT_KWARGS,
+}
+
+IOFORMAT = os.environ.get('IOFORMAT', IOFORMAT_KWARGS)
+
+
+def validate_io_format(v):
+    if callable(v):
+        return v
+    if v in IOFORMATS:
+        return v
+    raise ValidationError('Unsupported format {!r}.'.format(v))
+
 
 def check():
     if DEBUG and QUIET:
         raise RuntimeError('I cannot be verbose and quiet at the same time.')
+
+    if IOFORMAT not in IOFORMATS:
+        raise RuntimeError('Invalid default input/output format.')
+