I agree with @alexaryn in that responsibility of closing a stream should rest purely on the caller of PdfWriter, and not PdfWriter itself. Calling pdf_writer.close() or using the context manager should only clean up the resources that PdfWriter itself has created as part of its operation. I apologize for not catching the current behavior when I reviewed #1193. 😬

Looking at the code, there is also the inefficiency where PdfWriter.__init__ is called twice. It's also permissible to call PdfWriter with a PdfReader as the first argument (to simplify cloning), but in doing so with contexts, will hit the following error:

>>> with PdfReader("./sample-files/001-trivial/minimal-document.pdf") as reader:
...   with PdfWriter(reader) as writer:
...     print(writer.fileobj)
...
__init__
__enter__
__init__
<pypdf._reader.PdfReader object at 0x10304e970>
Traceback (most recent call last):
  File "<stdin>", line 4, in <module>
  File "/Users/mpeveler/code/github/pypdf/pypdf/_writer.py", line 373, in __exit__
    self.write(self.fileobj)
  File "/Users/mpeveler/code/github/pypdf/pypdf/_writer.py", line 1396, in write
    self.write_stream(stream)
  File "/Users/mpeveler/code/github/pypdf/pypdf/_writer.py", line 1367, in write_stream
    object_positions, free_objects = self._write_pdf_structure(stream)
  File "/Users/mpeveler/code/github/pypdf/pypdf/_writer.py", line 1500, in _write_pdf_structure
    stream.write(self.pdf_header.encode() + b"\n")
AttributeError: 'PdfReader' object has no attribute 'write'

I'm not sure what the correct behavior here should be, especially with regards to cloning. Like, if I call:

with PdfWriter("foo.pdf") as writer:

and foo.pdf exists, do I expect that it'll be cloned into the writer? I'd think no? However, I think this does force the second __init__ call to be made as there's no way to tell in the first __init__ that we're in a context. I'd think this could be confusing for end users, but I guess not since no one has written in about this behavior.

Originally posted by @MasterOdin in #2905 (comment)