"lambda" is not allowed to use as keyword arguments because it is a reserved word.

tkamishima · tkamishima · commit 02b966c84c42 · 2016-07-04T23:05:36.000+09:00
diff --git a/numpy/lib/function_base.py b/numpy/lib/function_base.py
@@ -1247,8 +1247,8 @@ def piecewise(x, condlist, funclist, *args, **kw):
     kw : dict, optional
         Keyword arguments used in calling `piecewise` are passed to the
         functions upon execution, i.e., if called
-        ``piecewise(..., ..., lambda=1)``, then each function is called as
-        ``f(x, lambda=1)``.
+        ``piecewise(..., ..., alpha=1)``, then each function is called as
+        ``f(x, alpha=1)``.
 
     Returns
     -------
