feat: support writing metadata

src/packaging/metadata.py

@@ -281,6 +281,43 @@ def _get_payload(msg: email.message.Message, source: bytes | str) -> str:
 _RAW_TO_EMAIL_MAPPING = {raw: email for email, raw in _EMAIL_TO_RAW_MAPPING.items()}
 
 
+# This class is for writing RFC822 messages
+class RFC822Policy(email.policy.EmailPolicy):
+    """
+    This is :class:`email.policy.EmailPolicy`, but with a simple ``header_store_parse``
+    implementation that handles multiline values, and some nice defaults.
+    """
+
+    utf8 = True
+    mangle_from_ = False
+    max_line_length = 0
+
+    def header_store_parse(self, name: str, value: str) -> tuple[str, str]:
+        size = len(name) + 2
+        value = value.replace("\n", "\n" + " " * size)
+        return (name, value)
+
+
+# This class is for writing RFC822 messages
+class RFC822Message(email.message.EmailMessage):
+    """
+    This is :class:`email.message.EmailMessage` with two small changes: it defaults to
+    our `RFC822Policy`, and it correctly writes unicode when being called
+    with `bytes()`.
+    """
+
+    def __init__(self) -> None:
+        super().__init__(policy=RFC822Policy())
+
+    def as_bytes(
+        self, unixfrom: bool = False, policy: email.policy.Policy | None = None
+    ) -> bytes:
+        """
+        This handles unicode encoding.
+        """
+        return self.as_string(unixfrom, policy=policy).encode("utf-8")
+
+
 def parse_email(data: bytes | str) -> tuple[RawMetadata, dict[str, list[str]]]:
     """Parse a distribution's metadata stored as email headers (e.g. from ``METADATA``).
 
@@ -859,3 +896,35 @@ def from_email(cls, data: bytes | str, *, validate: bool = True) -> Metadata:
     """``Provides`` (deprecated)"""
     obsoletes: _Validator[list[str] | None] = _Validator(added="1.1")
     """``Obsoletes`` (deprecated)"""
+
+    def as_rfc822(self) -> RFC822Message:
+        """
+        Return an RFC822 message with the metadata.
+        """
+        message = RFC822Message()
+        self._write_metadata(message)
+        return message
+
+    def _write_metadata(self, message: RFC822Message) -> None:
+        """
+        Return an RFC822 message with the metadata.
+        """
+        for name, validator in self.__class__.__dict__.items():
+            if isinstance(validator, _Validator) and name != "description":
+                value = getattr(self, name)
+                email_name = _RAW_TO_EMAIL_MAPPING[name]
+                if value is not None:
+                    if email_name == "project-url":
+                        for label, url in value.items():
+                            message[email_name] = f"{label}, {url}"
+                    elif email_name == "keywords":
+                        message[email_name] = ",".join(value)
+                    elif isinstance(value, list):
+                        for item in value:
+                            message[email_name] = str(item)
+                    else:
+                        message[email_name] = str(value)
+
+        # The description is a special case because it is in the body of the message.
+        if self.description is not None:
+            message.set_payload(self.description)