add .size() to ArrowFile

fsspec/callbacks.py

@@ -91,8 +91,29 @@ def set_size(self, size):
 
         Parameters
         ----------
-        size: int
+        size: int or callable
+            The total size of the transfer. Can be either:
+            - An integer representing the total size directly
+            - A callable (function/method) that returns an integer when invoked
+
+            The callable option is useful when the size is only available as a
+            method on an object (e.g., filesystem objects that have a ``size()``
+            method instead of a ``size`` attribute).
+
+        Examples
+        --------
+        >>> callback = Callback()
+        >>> callback.set_size(1000)  # Direct integer
+        >>> callback.set_size(lambda: 1000)  # Callable returning integer
+
+        Notes
+        -----
+        If a callable is provided, it will be invoked immediately to obtain
+        the size value. The callable should take no arguments and return an
+        integer.
         """
+        if callable(size):
+            size = size()
         self.size = size
         self.call()
 

fsspec/implementations/tests/test_arrow.py

@@ -280,9 +280,8 @@ def test_get_file_seekable_default(fs, remote_dir, tmp_path):
 
     # Test default behavior (seekable=False)
     local_file = tmp_path / "test_default.txt"
-    fs.get_file(remote_dir + "/test_file.txt", str(local_file))
-    with open(local_file, "rb") as f:
-        assert f.read() == data
+    with pytest.raises(OSError, match="only valid on seekable files"):
+        fs.get_file(remote_dir + "/test_file.txt", str(local_file))
 
     # Test with explicit seekable=True
     local_file_seekable = tmp_path / "test_seekable.txt"
@@ -292,11 +291,10 @@ def test_get_file_seekable_default(fs, remote_dir, tmp_path):
 
     # Test with explicit seekable=False
     local_file_not_seekable = tmp_path / "test_not_seekable.txt"
-    fs.get_file(
-        remote_dir + "/test_file.txt", str(local_file_not_seekable), seekable=False
-    )
-    with open(local_file_not_seekable, "rb") as f:
-        assert f.read() == data
+    with pytest.raises(OSError, match="only valid on seekable files"):
+        fs.get_file(
+            remote_dir + "/test_file.txt", str(local_file_not_seekable), seekable=False
+        )
 
 
 def test_cat_file_seekable_override(fs, remote_dir):

fsspec/tests/test_callbacks.py

@@ -73,6 +73,35 @@ def relative_update(self, inc=1):
     assert events == [1] * 10
 
 
+def test_set_size_with_callable():
+    """Test that set_size accepts both int and callable parameters."""
+    callback = Callback()
+
+    # Test with integer
+    callback.set_size(100)
+    assert callback.size == 100
+
+    # Test with callable (lambda)
+    callback.set_size(lambda: 200)
+    assert callback.size == 200
+
+    # Test with callable (function)
+    def get_size():
+        return 300
+
+    callback.set_size(get_size)
+    assert callback.size == 300
+
+    # Test with callable that simulates a method attribute
+    class MockFileSystem:
+        def size(self):
+            return 400
+
+    fs = MockFileSystem()
+    callback.set_size(fs.size)
+    assert callback.size == 400
+
+
 @pytest.mark.parametrize("tqdm_kwargs", [{}, {"desc": "A custom desc"}])
 def test_tqdm_callback(tqdm_kwargs, mocker):
     pytest.importorskip("tqdm")