Turn some conditinal logic not covered into asserts.

I'm not sure how to cover them

Author: Carreau

zarr/util.py

@@ -540,10 +540,8 @@ class PartialReadBuffer:
     def __init__(self, store_key, chunk_store):
         self.chunk_store = chunk_store
         # is it fsstore or an actual fsspec map object
-        if getattr(self.chunk_store, "map", None):
-            self.map = self.chunk_store.map
-        else:
-            self.map = self.chunk_store
+        assert hasattr(self, chunk_store, "map")
+        self.map = self.chunk_store.map
         self.fs = self.chunk_store.fs
         self.store_key = store_key
         self.key_path = self.map._key_to_str(store_key)
@@ -555,8 +553,7 @@ def __init__(self, store_key, chunk_store):
         self.read_blocks = set()
 
     def prepare_chunk(self):
-        if self.buff:
-            return
+        assert self.buff is None
         header = self.fs.read_block(self.key_path, 0, 16)
         nbytes, self.cbytes, blocksize = cbuffer_sizes(header)
         typesize, _shuffle, _memcpyd = cbuffer_metainfo(header)
@@ -582,8 +579,7 @@ def prepare_chunk(self):
         self.n_per_block = blocksize / typesize
 
     def read_part(self, start, nitems):
-        if not self.buff:
-            self.prepare_chunk()
+        assert self.buff is not None
         if self.nblocks == 1:
             return
         blocks_to_decompress = nitems / self.n_per_block