Set default cache type to "readahead" (#678)

See analysis by @sneha5gsm in #677

s3fs/core.py

@@ -205,10 +205,10 @@ class S3FileSystem(AsyncFileSystem):
     default_fill_cache : Bool (True)
         Whether to use cache filling with open by default. Refer to
         ``S3File.open``.
-    default_cache_type : string ('bytes')
+    default_cache_type : string ("readahead")
         If given, the default cache_type value used for ``open()``. Set to "none"
         if no caching is desired. See fsspec's documentation for other available
-        cache_type values. Default cache_type is 'bytes'.
+        cache_type values. Default cache_type is "readahead".
     version_aware : bool (False)
         Whether to support bucket versioning.  If enable this will require the
         user to have the necessary IAM permissions for dealing with versioned
@@ -265,7 +265,7 @@ def __init__(
         requester_pays=False,
         default_block_size=None,
         default_fill_cache=True,
-        default_cache_type="bytes",
+        default_cache_type="readahead",
         version_aware=False,
         config_kwargs=None,
         s3_additional_kwargs=None,
@@ -1972,7 +1972,7 @@ def __init__(
         fill_cache=True,
         s3_additional_kwargs=None,
         autocommit=True,
-        cache_type="bytes",
+        cache_type="readahead",
         requester_pays=False,
         cache_options=None,
     ):

s3fs/tests/test_s3fs.py

@@ -183,7 +183,7 @@ def test_simple(s3):
         assert out == data
 
 
-@pytest.mark.parametrize("default_cache_type", ["none", "bytes", "mmap"])
+@pytest.mark.parametrize("default_cache_type", ["none", "bytes", "mmap", "readahead"])
 def test_default_cache_type(s3, default_cache_type):
     data = b"a" * (10 * 2**20)
     s3 = S3FileSystem(
@@ -1024,7 +1024,7 @@ async def head_object(*args, **kwargs):
 
 def test_read_small(s3):
     fn = test_bucket_name + "/2014-01-01.csv"
-    with s3.open(fn, "rb", block_size=10) as f:
+    with s3.open(fn, "rb", block_size=10, cache_type="bytes") as f:
         out = []
         while True:
             data = f.read(3)