(All of this info applies to legacy PyPI, not Warehouse)
This is the code which twine uses to do uploads [0]
The URL is just the unmodified PyPI endpoint url, e.g. https://upload.pypi.org/legacy/, https://test.pypi.org/legacy/. Nothing special going on there.
At the point where it is shoved into the multipart encoder [1], this is what the raw set of tuples looks like
(sdist)
[('name', 'pulp-python'), ('version', '3.0.0a1.dev0'), ('filetype', 'sdist'), ('pyversion', None), ('metadata_version', '1.0'), ('summary', 'pulp-python plugin for the Pulp Project'), ('home_page', 'http://www.pulpproject.org'), ('author', 'Pulp Project Developers'), ('author_email', 'pulp-list@redhat.com'), ('maintainer', None), ('maintainer_email', None), ('license', 'GPLv2+'), ('description', None), ('keywords', None), ('platform', 'UNKNOWN'), ('download_url', None), ('comment', None), ('md5_digest', '1488f866e0a86455e3a90ed8152167bb'), ('sha256_digest', '41b0233eb20db4324c0285720f3c206b78df3219d6e636c09e85cfa22751d857'), ('blake2_256_digest', '9445dbe404dd962f9af7bd915b7e8bd92bd602fa032ec42c5ac33a9c5f6d4cc2'), ('requires_python', None), (':action', 'file_upload'), ('protcol_version', '1'), ('content', ('pulp-python-3.0.0a1.dev0.tar.gz', <_io.BufferedReader name='dist/pulp-python-3.0.0a1.dev0.tar.gz'>, 'application/octet-stream'))]
(bdist_wheel)
[('name', 'pulp-python'), ('version', '3.0.0a1.dev0'), ('filetype', 'bdist_wheel'), ('pyversion', 'py3'), ('metadata_version', '2.0'), ('summary', 'pulp-python plugin for the Pulp Project'), ('home_page', 'http://www.pulpproject.org'), ('author', 'Pulp Project Developers'), ('author_email', 'pulp-list@redhat.com'), ('maintainer', None), ('maintainer_email', None), ('license', 'GPLv2+'), ('description', 'UNKNOWN\n\n\n'), ('keywords', None), ('platform', 'UNKNOWN'), ('download_url', None), ('comment', None), ('md5_digest', 'e7589d3c306f46003bcbb90107b16421'), ('sha256_digest', '880c97d59ec6a94a5e35ef49cfeb3be7161d503dc7a7a283894b61bb4b5aacc5'), ('blake2_256_digest', '8848709ab5c62da72825b76477483073e2179e1681c41c8fa9545b18bf7ef93d'), ('requires_dist', 'pulpcore-plugin'), ('requires_python', None), (':action', 'file_upload'), ('protcol_version', '1'), ('content', ('pulp_python-3.0.0a1.dev0-py3-none-any.whl', <_io.BufferedReader name='dist/pulp_python-3.0.0a1.dev0-py3-none-any.whl'>, 'application/octet-stream'))]
This is the raw bytes content of the encoder itself are attached as files. One is an upload with an sdist content and one is an upload with bdist_wheel as the content. The request is slightly different between the two. This is the closest I could get to the actual HTTP request, considering PyPI is HTTPS-only which prevents actual interception of the request (i.e. Wireshark). It's good enough for our purposes.
The POST request is created by the requests library with these options [2]. The Content-Type header is
multipart/form-data; boundary=b9762ceb450a48d98e81d94d363782c0
Where "boundary" is a uuid generated here [3]
Looks like there is also a defect in this code... [4] "protocol" is misspelled, and it is spelled correctly in the register code above. I submitted a PR to twine for this.
[0] https://github.com/pypa/twine/blob/a90be8f57f02630c25cbb7e9f3d9a89578122f6c/twine/repository.py#L120-L172
[1] https://github.com/pypa/twine/blob/a90be8f57f02630c25cbb7e9f3d9a89578122f6c/twine/repository.py#L133
[2] https://github.com/pypa/twine/blob/a90be8f57f02630c25cbb7e9f3d9a89578122f6c/twine/repository.py#L145
[3] https://github.com/requests/toolbelt/blob/master/requests_toolbelt/multipart/encoder.py#L83
[4] https://github.com/pypa/twine/blob/master/twine/repository.py#L127
.