Check parameter consistensy in signal.iirdesign (#12061)

* Check if wp,ws are consistent

Ref. #11567

* Update documentation with a little explanation

...and add a colon after 'if'

* Minor changes

* Add tests for exceptions

* Fix pyflakes in new test

* Add new consistensy check and test

scipy/signal/filter_design.py

@@ -2075,8 +2075,10 @@ def iirdesign(wp, ws, gpass, gstop, analog=False, ftype='ellip', output='ba',
 
     Parameters
     ----------
-    wp, ws : float
-        Passband and stopband edge frequencies.
+    wp, ws : float or array like, shape (2,)
+        Passband and stopband edge frequencies. Possible values are scalars
+        (for lowpass and highpass filters) or ranges (for bandpass and bandstop
+        filters).
         For digital filters, these are in the same units as `fs`. By default,
         `fs` is 2 half-cycles/sample, so these are normalized from 0 to 1,
         where 1 is the Nyquist frequency. For example:
@@ -2087,6 +2089,8 @@ def iirdesign(wp, ws, gpass, gstop, analog=False, ftype='ellip', output='ba',
             - Bandstop:  wp = [0.1, 0.6],   ws = [0.2, 0.5]
 
         For analog filters, `wp` and `ws` are angular frequencies (e.g., rad/s).
+        Note, that for bandpass and bandstop filters passband must lie strictly
+        inside stopband or vice versa.
     gpass : float
         The maximum loss in the passband (dB).
     gstop : float
@@ -2183,6 +2187,18 @@ def iirdesign(wp, ws, gpass, gstop, analog=False, ftype='ellip', output='ba',
     wp = atleast_1d(wp)
     ws = atleast_1d(ws)
 
+    if wp.shape[0] != ws.shape[0] or wp.shape not in [(1,), (2,)]:
+        raise ValueError("wp and ws must have one or two elements each, and"
+                         "the same shape, got %s and %s"
+                         % (wp.shape, ws.shape))
+    if wp.shape[0] == 2:
+        if wp[0] < 0 or ws[0] < 0:
+            raise ValueError("Values for wp, ws can't be negative")
+        if not((ws[0] < wp[0] and wp[1] < ws[1]) or
+            (wp[0] < ws[0] and ws[1] < wp[1])):
+            raise ValueError("Passband must lie strictly inside stopband"
+                         " or vice versa")
+
     band_type = 2 * (len(wp) - 1)
     band_type += 1
     if wp[0] >= ws[0]:

scipy/signal/tests/test_filter_design.py

@@ -14,9 +14,9 @@
                           butter, buttord, cheb1ap, cheb1ord, cheb2ap,
                           cheb2ord, cheby1, cheby2, ellip, ellipap, ellipord,
                           firwin, freqs_zpk, freqs, freqz, freqz_zpk,
-                          group_delay, iirfilter, iirnotch, iirpeak, lp2bp,
-                          lp2bs, lp2hp, lp2lp, normalize, sos2tf, sos2zpk,
-                          sosfreqz, tf2sos, tf2zpk, zpk2sos, zpk2tf,
+                          group_delay, iirdesign, iirfilter, iirnotch, iirpeak,
+                          lp2bp, lp2bs, lp2hp, lp2lp, normalize, sos2tf,
+                          sos2zpk, sosfreqz, tf2sos, tf2zpk, zpk2sos, zpk2tf,
                           bilinear_zpk, lp2lp_zpk, lp2hp_zpk, lp2bp_zpk,
                           lp2bs_zpk)
 from scipy.signal.filter_design import (_cplxreal, _cplxpair, _norm_factor,
@@ -3611,6 +3611,26 @@ def test_fs_param(self):
         assert_allclose(abs(hp[2]), 1, rtol=1e-10)
 
 
+class TestIIRDesign(object):
+
+    def test_exceptions(self):
+        with pytest.raises(ValueError, match="the same shape"):
+            iirdesign(0.2, [0.1, 0.3], 1, 40)
+        with pytest.raises(ValueError, match="the same shape"):
+            iirdesign(np.array([[0.3, 0.6], [0.3, 0.6]]),
+                      np.array([[0.4, 0.5], [0.4, 0.5]]), 1, 40)
+        with pytest.raises(ValueError, match="can't be negative"):
+            iirdesign([0.1, 0.3], [-0.1, 0.5], 1, 40)
+        with pytest.raises(ValueError, match="strictly inside stopband"):
+            iirdesign([0.1, 0.4], [0.5, 0.6], 1, 40)
+        with pytest.raises(ValueError, match="strictly inside stopband"):
+            iirdesign([0.5, 0.6], [0.1, 0.4], 1, 40)
+        with pytest.raises(ValueError, match="strictly inside stopband"):
+            iirdesign([0.3, 0.6], [0.4, 0.7], 1, 40)
+        with pytest.raises(ValueError, match="strictly inside stopband"):
+            iirdesign([0.4, 0.7], [0.3, 0.6], 1, 40)
+
+
 class TestIIRFilter(object):
 
     def test_symmetry(self):