19.1.4. email.policy: Policy Objects

Source code: Lib/email/policy.py

The email package’s prime focus is the handling of email messages as described by the various email and MIME RFCs. However, the general format of email messages (a block of header fields each consisting of a name followed by a colon followed by a value, the whole block followed by a blank line and an arbitrary ‘body’), is a format that has found utility outside of the realm of email. Some of these uses conform fairly closely to the main RFCs, some do not. And even when working with email, there are times when it is desirable to break strict compliance with the RFCs.

Policy objects give the email package the flexibility to handle all these disparate use cases.

A Policy object encapsulates a set of attributes and methods that control the behavior of various components of the email package during use. Policy instances can be passed to various classes and methods in the email package to alter the default behavior. The settable values and their defaults are described below.

There is a default policy used by all classes in the email package. This policy is named Compat32, with a corresponding pre-defined instance named compat32. It provides for complete backward compatibility (in some cases, including bug compatibility) with the pre-Python3.3 version of the email package.

The first part of this documentation covers the features of Policy, an abstract base class that defines the features that are common to all policy objects, including compat32. This includes certain hook methods that are called internally by the email package, which a custom policy could override to obtain different behavior.

When a Message object is created, it acquires a policy. By default this will be compat32, but a different policy can be specified. If the Message is created by a parser, a policy passed to the parser will be the policy used by the Message it creates. If the Message is created by the program, then the policy can be specified when it is created. When a Message is passed to a generator, the generator uses the policy from the Message by default, but you can also pass a specific policy to the generator that will override the one stored on the Message object.

Policy instances are immutable, but they can be cloned, accepting the same keyword arguments as the class constructor and returning a new Policy instance that is a copy of the original but with the specified attributes values changed.

As an example, the following code could be used to read an email message from a file on disk and pass it to the system sendmail program on a Unix system:

>>> from email import message_from_binary_file
>>> from email.generator import BytesGenerator
>>> from email import policy
>>> from subprocess import Popen, PIPE
>>> with open('mymsg.txt', 'rb') as f:
...     msg = message_from_binary_file(f, policy=policy.default)
>>> p = Popen(['sendmail', msg['To'].addresses[0]], stdin=PIPE)
>>> g = BytesGenerator(p.stdin, policy=msg.policy.clone(linesep='\r\n'))
>>> g.flatten(msg)
>>> p.stdin.close()
>>> rc = p.wait()

Here we are telling BytesGenerator to use the RFC correct line separator characters when creating the binary string to feed into sendmail's stdin, where the default policy would use \n line separators.

Some email package methods accept a policy keyword argument, allowing the policy to be overridden for that method. For example, the following code uses the as_bytes() method of the msg object from the previous example and writes the message to a file using the native line separators for the platform on which it is running:

>>> import os
>>> with open('converted.txt', 'wb') as f:
...     f.write(msg.as_bytes(policy=msg.policy.clone(linesep=os.linesep)))
17

Policy objects can also be combined using the addition operator, producing a policy object whose settings are a combination of the non-default values of the summed objects:

>>> compat_SMTP = policy.compat32.clone(linesep='\r\n')
>>> compat_strict = policy.compat32.clone(raise_on_defect=True)
>>> compat_strict_SMTP = compat_SMTP + compat_strict

This operation is not commutative; that is, the order in which the objects are added matters. To illustrate:

>>> policy100 = policy.compat32.clone(max_line_length=100)
>>> policy80 = policy.compat32.clone(max_line_length=80)
>>> apolicy = policy100 + policy80
>>> apolicy.max_line_length
80
>>> apolicy = policy80 + policy100
>>> apolicy.max_line_length
100

class email.policy.Policy(**kw)

This is the abstract base class for all policy classes. It provides default implementations for a couple of trivial methods, as well as the implementation of the immutability property, the clone() method, and the constructor semantics.

The constructor of a policy class can be passed various keyword arguments. The arguments that may be specified are any non-method properties on this class, plus any additional non-method properties on the concrete class. A value specified in the constructor will override the default value for the corresponding attribute.

This class defines the following properties, and thus values for the following may be passed in the constructor of any policy class:

max_line_length

The maximum length of any line in the serialized output, not counting the end of line character(s). Default is 78, per RFC 5322. A value of 0 or None indicates that no line wrapping should be done at all.

linesep

The string to be used to terminate lines in serialized output. The default is \n because that’s the internal end-of-line discipline used by Python, though \r\n is required by the RFCs.

cte_type

Controls the type of Content Transfer Encodings that may be or are required to be used. The possible values are:

7bit	all data must be “7 bit clean” (ASCII-only). This means that where necessary data will be encoded using either quoted-printable or base64 encoding.
8bit	data is not constrained to be 7 bit clean. Data in headers is still required to be ASCII-only and so will be encoded (see ‘binary_fold’ below for an exception), but body parts may use the 8bit CTE.

A cte_type value of 8bit only works with BytesGenerator, not Generator, because strings cannot contain binary data. If a Generator is operating under a policy that specifies cte_type=8bit, it will act as if cte_type is 7bit.

raise_on_defect

If True, any defects encountered will be raised as errors. If False (the default), defects will be passed to the register_defect() method.

mangle_from_

If True, lines starting with “From “ in the body are escaped by putting a > in front of them. This parameter is used when the message is being serialized by a generator. Default: False.

New in version 3.5: The mangle_from_ parameter.

The following Policy method is intended to be called by code using the email library to create policy instances with custom settings:

clone(**kw)

Return a new Policy instance whose attributes have the same values as the current instance, except where those attributes are given new values by the keyword arguments.

The remaining Policy methods are called by the email package code, and are not intended to be called by an application using the email package. A custom policy must implement all of these methods.

handle_defect(obj, defect)

Handle a defect found on obj. When the email package calls this method, defect will always be a subclass of Defect.

The default implementation checks the raise_on_defect flag. If it is True, defect is raised as an exception. If it is False (the default), obj and defect are passed to register_defect().

register_defect(obj, defect)

Register a defect on obj. In the email package, defect will always be a subclass of Defect.

The default implementation calls the append method of the defects attribute of obj. When the email package calls handle_defect, obj will normally have a defects attribute that has an append method. Custom object types used with the email package (for example, custom Message objects) should also provide such an attribute, otherwise defects in parsed messages will raise unexpected errors.

header_max_count(name)

Return the maximum allowed number of headers named name.

Called when a header is added to a Message object. If the returned value is not 0 or None, and there are already a number of headers with the name name equal to the value returned, a ValueError is raised.

Because the default behavior of Message.__setitem__ is to append the value to the list of headers, it is easy to create duplicate headers without realizing it. This method allows certain headers to be limited in the number of instances of that header that may be added to a Message programmatically. (The limit is not observed by the parser, which will faithfully produce as many headers as exist in the message being parsed.)

The default implementation returns None for all header names.

header_source_parse(sourcelines)

The email package calls this method with a list of strings, each string ending with the line separation characters found in the source being parsed. The first line includes the field header name and separator. All whitespace in the source is preserved. The method should return the (name, value) tuple that is to be stored in the Message to represent the parsed header.

If an implementation wishes to retain compatibility with the existing email package policies, name should be the case preserved name (all characters up to the ‘:‘ separator), while value should be the unfolded value (all line separator characters removed, but whitespace kept intact), stripped of leading whitespace.

sourcelines may contain surrogateescaped binary data.

There is no default implementation

header_store_parse(name, value)

The email package calls this method with the name and value provided by the application program when the application program is modifying a Message programmatically (as opposed to a Message created by a parser). The method should return the (name, value) tuple that is to be stored in the Message to represent the header.

If an implementation wishes to retain compatibility with the existing email package policies, the name and value should be strings or string subclasses that do not change the content of the passed in arguments.

There is no default implementation

header_fetch_parse(name, value)

The email package calls this method with the name and value currently stored in the Message when that header is requested by the application program, and whatever the method returns is what is passed back to the application as the value of the header being retrieved. Note that there may be more than one header with the same name stored in the Message; the method is passed the specific name and value of the header destined to be returned to the application.

value may contain surrogateescaped binary data. There should be no surrogateescaped binary data in the value returned by the method.

There is no default implementation

fold(name, value)

The email package calls this method with the name and value currently stored in the Message for a given header. The method should return a string that represents that header “folded” correctly (according to the policy settings) by composing the name with the value and inserting linesep characters at the appropriate places. See RFC 5322 for a discussion of the rules for folding email headers.

value may contain surrogateescaped binary data. There should be no surrogateescaped binary data in the string returned by the method.

fold_binary(name, value)

The same as fold(), except that the returned value should be a bytes object rather than a string.

value may contain surrogateescaped binary data. These could be converted back into binary data in the returned bytes object.

class email.policy.Compat32(**kw)

This concrete Policy is the backward compatibility policy. It replicates the behavior of the email package in Python 3.2. The policy module also defines an instance of this class, compat32, that is used as the default policy. Thus the default behavior of the email package is to maintain compatibility with Python 3.2.

The following attributes have values that are different from the Policy default:

mangle_from_

The default is True.

The class provides the following concrete implementations of the abstract methods of Policy:

header_source_parse(sourcelines)

The name is parsed as everything up to the ‘:‘ and returned unmodified. The value is determined by stripping leading whitespace off the remainder of the first line, joining all subsequent lines together, and stripping any trailing carriage return or linefeed characters.

header_store_parse(name, value)

The name and value are returned unmodified.

header_fetch_parse(name, value)

If the value contains binary data, it is converted into a Header object using the unknown-8bit charset. Otherwise it is returned unmodified.

fold(name, value)

Headers are folded using the Header folding algorithm, which preserves existing line breaks in the value, and wraps each resulting line to the max_line_length. Non-ASCII binary data are CTE encoded using the unknown-8bit charset.

fold_binary(name, value)

Headers are folded using the Header folding algorithm, which preserves existing line breaks in the value, and wraps each resulting line to the max_line_length. If cte_type is 7bit, non-ascii binary data is CTE encoded using the unknown-8bit charset. Otherwise the original source header is used, with its existing line breaks and any (RFC invalid) binary data it may contain.

An instance of Compat32 is provided as a module constant:

email.policy.compat32

An instance of Compat32, providing backward compatibility with the behavior of the email package in Python 3.2.

class email.policy.EmailPolicy(**kw)

This concrete Policy provides behavior that is intended to be fully compliant with the current email RFCs. These include (but are not limited to) RFC 5322, RFC 2047, and the current MIME RFCs.

This policy adds new header parsing and folding algorithms. Instead of simple strings, headers are str subclasses with attributes that depend on the type of the field. The parsing and folding algorithm fully implement RFC 2047 and RFC 5322.

In addition to the settable attributes listed above that apply to all policies, this policy adds the following additional attributes:

utf8

If False, follow RFC 5322, supporting non-ASCII characters in headers by encoding them as “encoded words”. If True, follow RFC 6532 and use utf-8 encoding for headers. Messages formatted in this way may be passed to SMTP servers that support the SMTPUTF8 extension (RFC 6531).

refold_source

If the value for a header in the Message object originated from a parser (as opposed to being set by a program), this attribute indicates whether or not a generator should refold that value when transforming the message back into stream form. The possible values are:

none	all source values use original folding
long	source values that have any line that is longer than max_line_length will be refolded
all	all values are refolded.

The default is long.

header_factory

A callable that takes two arguments, name and value, where name is a header field name and value is an unfolded header field value, and returns a string subclass that represents that header. A default header_factory (see headerregistry) is provided that understands some of the RFC 5322 header field types. (Currently address fields and date fields have special treatment, while all other fields are treated as unstructured. This list will be completed before the extension is marked stable.)

content_manager

An object with at least two methods: get_content and set_content. When the get_content() or set_content() method of a Message object is called, it calls the corresponding method of this object, passing it the message object as its first argument, and any arguments or keywords that were passed to it as additional arguments. By default content_manager is set to raw_data_manager.

New in version 3.4.

The class provides the following concrete implementations of the abstract methods of Policy:

header_max_count(name)

Returns the value of the max_count attribute of the specialized class used to represent the header with the given name.

header_source_parse(sourcelines)

The implementation of this method is the same as that for the Compat32 policy.

header_store_parse(name, value)

The name is returned unchanged. If the input value has a name attribute and it matches name ignoring case, the value is returned unchanged. Otherwise the name and value are passed to header_factory, and the resulting header object is returned as the value. In this case a ValueError is raised if the input value contains CR or LF characters.

header_fetch_parse(name, value)

If the value has a name attribute, it is returned to unmodified. Otherwise the name, and the value with any CR or LF characters removed, are passed to the header_factory, and the resulting header object is returned. Any surrogateescaped bytes get turned into the unicode unknown-character glyph.

fold(name, value)

Header folding is controlled by the refold_source policy setting. A value is considered to be a ‘source value’ if and only if it does not have a name attribute (having a name attribute means it is a header object of some sort). If a source value needs to be refolded according to the policy, it is converted into a header object by passing the name and the value with any CR and LF characters removed to the header_factory. Folding of a header object is done by calling its fold method with the current policy.

Source values are split into lines using splitlines(). If the value is not to be refolded, the lines are rejoined using the linesep from the policy and returned. The exception is lines containing non-ascii binary data. In that case the value is refolded regardless of the refold_source setting, which causes the binary data to be CTE encoded using the unknown-8bit charset.

fold_binary(name, value)

The same as fold() if cte_type is 7bit, except that the returned value is bytes.

If cte_type is 8bit, non-ASCII binary data is converted back into bytes. Headers with binary data are not refolded, regardless of the refold_header setting, since there is no way to know whether the binary data consists of single byte characters or multibyte characters.

The following instances of EmailPolicy provide defaults suitable for specific application domains. Note that in the future the behavior of these instances (in particular the HTTP instance) may be adjusted to conform even more closely to the RFCs relevant to their domains.

email.policy.default

An instance of EmailPolicy with all defaults unchanged. This policy uses the standard Python \n line endings rather than the RFC-correct \r\n.

email.policy.SMTP

Suitable for serializing messages in conformance with the email RFCs. Like default, but with linesep set to \r\n, which is RFC compliant.

email.policy.SMTPUTF8

The same as SMTP except that utf8 is True. Useful for serializing messages to a message store without using encoded words in the headers. Should only be used for SMTP trasmission if the sender or recipient addresses have non-ASCII characters (the smtplib.SMTP.send_message() method handles this automatically).

email.policy.HTTP

Suitable for serializing headers with for use in HTTP traffic. Like SMTP except that max_line_length is set to None (unlimited).

email.policy.strict

Convenience instance. The same as default except that raise_on_defect is set to True. This allows any policy to be made strict by writing:

somepolicy + policy.strict

With all of these EmailPolicies, the effective API of the email package is changed from the Python 3.2 API in the following ways:

• Setting a header on a Message results in that header being parsed and a header object created.
• Fetching a header value from a Message results in that header being parsed and a header object created and returned.
• Any header object, or any header that is refolded due to the policy settings, is folded using an algorithm that fully implements the RFC folding algorithms, including knowing where encoded words are required and allowed.

From the application view, this means that any header obtained through the Message is a header object with extra attributes, whose string value is the fully decoded unicode value of the header. Likewise, a header may be assigned a new value, or a new header created, using a unicode string, and the policy will take care of converting the unicode string into the correct RFC encoded form.

The header objects and their attributes are described in headerregistry.