Lib/email/message.py (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/email/message.py ↗

This annotation covers the 3.6+ EmailMessage API (policy-aware) and multipart construction. See lib_email_detail for Message, headers, payload, and MIME basics.

Map

Lines	Symbol	Role
1-80	MIMEPart	Base for policy-aware message parts
81-200	get_body / get_content	Retrieve decoded body (text, html, attachments)
201-350	make_alternative	Construct multipart/alternative container
351-500	make_mixed	Construct multipart/mixed container
501-650	EmailMessage	Top-level message with convenience methods
651-800	as_string / as_bytes	Serialize to text/bytes using a Generator
801-1000	policy	EmailPolicy, SMTP, SMTPUTF8, HTTP policy objects

Reading

get_body

# CPython: Lib/email/message.py:430 MIMEPart.get_body
def get_body(self, preferencelist=('related', 'html', 'plain')):
    """Find the best text body part according to preference."""
    # Walks the MIME tree, preferring related > html > plain
    for pref in preferencelist:
        for part in self.walk():
            if part.get_content_maintype() == 'multipart':
                continue
            if part.get_content_subtype() == pref:
                return part
    return None

get_content

# CPython: Lib/email/message.py:480 MIMEPart.get_content
def get_content(self, *args, content_manager=None, **kw):
    """Return decoded content of this part."""
    if content_manager is None:
        content_manager = self.policy.content_manager
    return content_manager.get_content(self, *args, **kw)

The content manager handles charset decoding and base64/quoted-printable decoding transparently.

make_alternative

# CPython: Lib/email/message.py:530 MIMEPart.make_alternative
def make_alternative(self, _subtype='alternative', policy=None):
    """Convert this message into a multipart/alternative container."""
    keep_headers, payload = self._make_multipart(
        _subtype, [self], policy)
    del self._payload
    del self._headers[:]
    self._headers[:] = keep_headers
    self.attach(payload)

msg.make_alternative() is used when adding an HTML version to an existing plain-text message.

as_string

# CPython: Lib/email/message.py:680 Message.as_string
def as_string(self, unixfrom=False, maxheaderlen=None, policy=None):
    """Return the entire message as a string."""
    from email.generator import Generator
    policy = self.policy if policy is None else self.policy.clone(policy)
    fp = StringIO()
    g = Generator(fp, mangle_from_=False, maxheaderlen=maxheaderlen,
                  policy=policy)
    g.flatten(self, unixfrom=unixfrom)
    return fp.getvalue()

Policy

# CPython: Lib/email/_header_value_parser.py (policy anatomy)
# email.policy.default — modern API, utf-8 aware
# email.policy.SMTP    — CRLF line endings, max 998 chars/line
# email.policy.compat32 — legacy compat32 (old Message API default)

EmailMessage uses policy=email.policy.default by default.

gopy notes

email is pure Python. get_content calls ContentManager.get_content which uses policy.content_manager. as_bytes uses BytesGenerator, which calls write() on a BytesIO. All these are pure Python and work with gopy's io.StringIO / io.BytesIO.