sagemathgh-37781: SetPartition return empty iterator instead of err…

…oring out in degenerate cases

    
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Replaced the error raised by `SetPartition` in a degenerate case (see
below) by an empty iterator, to match the behaviour of similar functions
such as `Partitions` and `OrderedSetPartitions`. Old behaviour:
```Python
sage: list( Partitions(3,length=4) )
[]
sage: list( OrderedSetPartitions(range(3),4) )
[]
sage: list( SetPartitions(range(3),4) )
...
ValueError: part must be <= len(set)
```
New behaviour
```Python
sage: list( SetPartitions(range(3),4) )
[]
```
This solves issue sagemath#37643.

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### ⌛ Dependencies

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URL: sagemath#37781
Reported by: Nolord
Reviewer(s): Travis Scrimshaw

src/sage/combinat/set_partition.py

@@ -2034,6 +2034,14 @@ class SetPartitions(UniqueRepresentation, Parent):
         sage: SetPartitions('aabcd').cardinality()                                      # needs sage.libs.flint
         15
 
+    If the number of parts exceeds the length of the set,
+    an empty iterator is returned (:issue:`37643`)::
+
+        sage: SetPartitions(range(3), 4).list()
+        []
+        sage: SetPartitions('abcd', 6).list()
+        []
+
     REFERENCES:
 
     - :wikipedia:`Partition_of_a_set`
@@ -2062,18 +2070,16 @@ def __classcall_private__(cls, s=None, part=None):
                 pass
             s = frozenset(s)
 
-        if part is not None:
+        if part is None:
+            return SetPartitions_set(s)
+        else:
             if isinstance(part, (int, Integer)):
-                if len(s) < part:
-                    raise ValueError("part must be <= len(set)")
                 return SetPartitions_setn(s, part)
             else:
                 part = sorted(part, reverse=True)
                 if part not in Partitions(len(s)):
                     raise ValueError("part must be an integer partition of %s" % len(s))
                 return SetPartitions_setparts(s, Partition(part))
-        else:
-            return SetPartitions_set(s)
 
     def __contains__(self, x):
         """

src/sage/combinat/set_partition_iterator.pyx

@@ -130,5 +130,7 @@ def set_partition_iterator_blocks(base_set, Py_ssize_t k):
     cdef Py_ssize_t n = len(base)
     cdef list a = list(range(n))
     # TODO: implement _set_partition_block_gen as an iterative algorithm
+    if n < k:
+        return
     for P in _set_partition_block_gen(n, k, a):
         yield from_word(<list> P, base)