Use setdefault for ctx caches

Author: oscarbenjamin

src/flint/types/nmod.pyx

@@ -62,7 +62,7 @@ cdef class nmod_ctx:
         """Retrieve an nmod context from the cache or create a new one."""
         ctx = _nmod_ctx_cache.get(mod)
         if ctx is None:
-            _nmod_ctx_cache[mod] = ctx = nmod_ctx._new_ctx(mod)
+            ctx = _nmod_ctx_cache.setdefault(mod, nmod_ctx._new_ctx(mod))
         return ctx
 
     @staticmethod

src/flint/types/nmod_mat.pyx

@@ -94,7 +94,7 @@ cdef class nmod_mat_ctx:
         """Retrieve an nmod_mat context from the cache or create a new one."""
         ctx = _nmod_mat_ctx_cache.get(mod)
         if ctx is None:
-            _nmod_mat_ctx_cache[mod] = ctx = nmod_mat_ctx._new_ctx(mod)
+            ctx = _nmod_mat_ctx_cache.setdefault(mod, nmod_mat_ctx._new_ctx(mod))
         return ctx
 
     @staticmethod

src/flint/types/nmod_poly.pyx

@@ -49,7 +49,7 @@ cdef class nmod_poly_ctx:
         """Retrieve an nmod_poly context from the cache or create a new one."""
         ctx = _nmod_poly_ctx_cache.get(mod)
         if ctx is None:
-            _nmod_poly_ctx_cache[mod] = ctx = nmod_poly_ctx._new_ctx(mod)
+            ctx = _nmod_poly_ctx_cache.setdefault(mod, nmod_poly_ctx._new_ctx(mod))
         return ctx
 
     @staticmethod