py/objstringio: If created from immutable object, follow copy on write policy.

Don't create copy of immutable object's contents until .write() is called
on BytesIO.

1 files changed, 20 insertions, 0 deletions

diff --git a/tests/io/bytesio_cow.py b/tests/io/bytesio_cow.py
new file mode 100644
index 000000000..92654a000
--- /dev/null
+++ b/tests/io/bytesio_cow.py
@@ -0,0 +1,20 @@
+# Make sure that write operations on io.BytesIO don't
+# change original object it was constructed from.
+try:
+    import uio as io
+except ImportError:
+    import io
+
+b = b"foobar"
+
+a = io.BytesIO(b)
+a.write(b"1")
+print(b)
+print(a.getvalue())
+
+b = bytearray(b"foobar")
+
+a = io.BytesIO(b)
+a.write(b"1")
+print(b)
+print(a.getvalue())