override the signature of plotfunc (#4359)

* override the signature of plotfunc

* update whats-new.rst

* explain the reasons for the signature dummy

Author: keewis

doc/whats-new.rst

@@ -77,6 +77,7 @@ Bug fixes
   and :py:meth:`DataArray.str.wrap` (:issue:`4334`). By `Mathias Hauser <https://github.com/mathause>`_.
 - Fixed overflow issue causing incorrect results in computing means of :py:class:`cftime.datetime`
   arrays (:issue:`4341`). By `Spencer Clark <https://github.com/spencerkclark>`_.
+- fix the signature of the plot methods. (:pull:`4359`) By `Justus Magin <https://github.com/keewis>`_.
 - Fix :py:func:`xarray.apply_ufunc` with ``vectorize=True`` and ``exclude_dims`` (:issue:`3890`).
   By `Mathias Hauser <https://github.com/mathause>`_.
 - Fix `KeyError` when doing linear interpolation to an nd `DataArray`

xarray/plot/plot.py

@@ -463,6 +463,15 @@ def step(self, *args, **kwargs):
         return step(self._da, *args, **kwargs)
 
 
+def override_signature(f):
+    def wrapper(func):
+        func.__wrapped__ = f
+
+        return func
+
+    return wrapper
+
+
 def _plot2d(plotfunc):
     """
     Decorator for common 2d plotting logic
@@ -572,6 +581,16 @@ def _plot2d(plotfunc):
     # Build on the original docstring
     plotfunc.__doc__ = f"{plotfunc.__doc__}\n{commondoc}"
 
+    # plotfunc and newplotfunc have different signatures:
+    # - plotfunc: (x, y, z, ax, **kwargs)
+    # - newplotfunc: (darray, x, y, **kwargs)
+    # where plotfunc accepts numpy arrays, while newplotfunc accepts a DataArray
+    # and variable names. newplotfunc also explicitly lists most kwargs, so we
+    # need to shorten it
+    def signature(darray, x, y, **kwargs):
+        pass
+
+    @override_signature(signature)
     @functools.wraps(plotfunc)
     def newplotfunc(
         darray,