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Lib/email/mime/multipart.py

Source:

cpython 3.14 @ ab2d84fe1023/Lib/email/mime/multipart.py

Lib/email/mime/multipart.py defines MIMEMultipart, the class for constructing multipart/* MIME messages. It subclasses MIMEBase and provides attach() to add sub-parts. Boundary generation is deferred to the Generator layer at serialization time unless the caller supplies one explicitly.

Map​

LinesSymbolRole
1-10importsMIMEBase
12-100MIMEMultipartConstructor with subtype, boundary, subparts, and params

Reading​

Constructor​

MIMEMultipart.__init__ accepts _subtype (default mixed), an optional boundary string, an optional _subparts iterable, and keyword **_params for additional Content-Type parameters. It passes multipart and _subtype up to MIMEBase.__init__ which sets the Content-Type header.

# CPython: Lib/email/mime/multipart.py:12 MIMEMultipart.__init__
class MIMEMultipart(MIMEBase):
    def __init__(self, _subtype='mixed', boundary=None, _subparts=None,
                 *, policy=None, **_params):
        MIMEBase.__init__(self, 'multipart', _subtype,
                          policy=policy, **_params)
        if boundary is not None:
            self.set_boundary(boundary)
        if _subparts is not None:
            for p in _subparts:
                self.attach(p)

Attaching sub-parts​

attach() (inherited from Message) appends a part to self._payload. The payload starts as None for non-multipart messages; the first call to attach() on a MIMEMultipart converts it to a list.

# CPython: Lib/email/message.py:160 Message.attach
def attach(self, payload):
    if self._payload is None:
        self._payload = [payload]
    else:
        self._payload.append(payload)

Boundary generation​

If no boundary is passed to the constructor, set_boundary is never called, so self.get_boundary() returns None. The Generator layer calls _make_boundary() at write time to generate a unique boundary string based on time.time() and a random component. This avoids needing a boundary until the message is actually serialized.

Common subtypes​

multipart/mixed is the default for attaching unrelated parts (e.g., a text body and a file attachment). multipart/alternative signals that all parts are representations of the same content, with the preferred one last. multipart/related is used for HTML with embedded images.

gopy notes​

Not yet ported. The planned package path is module/email/mime/. The Go type would hold a []*Message payload slice and generate the boundary lazily in a WriteTo method.