BUG: optimize: make curve_fit work with method as callable. Closes #1531

.

scipy/optimize/minpack.py

@@ -407,16 +407,20 @@ def curve_fit(f, xdata, ydata, p0=None, sigma=None, **kw):
     >>> popt, pcov = curve_fit(func, x, yn)
 
     """
-    if p0 is None or isscalar(p0):
+    if p0 is None:
         # determine number of parameters by inspecting the function
         import inspect
         args, varargs, varkw, defaults = inspect.getargspec(f)
         if len(args) < 2:
             msg = "Unable to determine number of fit parameters."
             raise ValueError(msg)
-        if p0 is None:
-            p0 = 1.0
-        p0 = [p0]*(len(args)-1)
+        if 'self' in args:
+            p0 = [1.0] * (len(args)-2)
+        else:
+            p0 = [1.0] * (len(args)-1)
+
+    if isscalar(p0):
+        p0 = array([p0])
 
     args = (xdata, ydata, f)
     if sigma is None:

scipy/optimize/tests/test_minpack.py

@@ -221,7 +221,23 @@ def func(x, a, b):
         assert_(len(popt) == 2)
         assert_(pcov.shape == (2,2))
         assert_array_almost_equal(popt, [1.7989, 1.1642], decimal=4)
-        assert_array_almost_equal(pcov, [[0.0852, -0.1260],[-0.1260, 0.1912]], decimal=4)
+        assert_array_almost_equal(pcov, [[0.0852, -0.1260],[-0.1260, 0.1912]],
+                                  decimal=4)
+
+    def test_func_is_classmethod(self):
+        class test_self(object):
+            """This class tests if curve_fit passes the correct number of
+               arguments when the model function is a class instance method.
+            """
+            def func(self, x, a, b):
+                return b * x**a
+
+        test_self_inst = test_self()
+        popt, pcov = curve_fit(test_self_inst.func, self.x, self.y)
+        assert_(pcov.shape == (2,2))
+        assert_array_almost_equal(popt, [1.7989, 1.1642], decimal=4)
+        assert_array_almost_equal(pcov, [[0.0852, -0.1260], [-0.1260, 0.1912]],
+                                  decimal=4)
 
 
 class TestFixedPoint(TestCase):