Document ImageFile.MAXBLOCK (#9163)

Author: hugovk

docs/reference/ImageFile.rst

@@ -74,5 +74,6 @@ Constants
 ---------
 
 .. autodata:: PIL.ImageFile.LOAD_TRUNCATED_IMAGES
+.. autodata:: PIL.ImageFile.MAXBLOCK
 .. autodata:: PIL.ImageFile.ERRORS
     :annotation:

src/PIL/ImageFile.py

@@ -46,6 +46,18 @@
 logger = logging.getLogger(__name__)
 
 MAXBLOCK = 65536
+"""
+By default, Pillow processes image data in blocks. This helps to prevent excessive use
+of resources. Codecs may disable this behaviour with ``_pulls_fd`` or ``_pushes_fd``.
+
+When reading an image, this is the number of bytes to read at once.
+
+When writing an image, this is the number of bytes to write at once.
+If the image width times 4 is greater, then that will be used instead.
+Plugins may also set a greater number.
+
+User code may set this to another number.
+"""
 
 SAFEBLOCK = 1024 * 1024