Trying to change values (fe. "To", "From") of email.mime.text.MIMEText after initial assigment silently doesn't change them.

[ukaszKaptur]

BPO	36303
Nosy	@warsaw, @bitdancer, @remilapeyre, @Łukasz Kaptur

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-03-15.15:10:43.375>
created_at = <Date 2019-03-15.13:15:00.499>
labels = ['3.8', 'type-bug', '3.7', 'expert-email', 'invalid']
title = 'Trying to change values (fe. "To", "From") of email.mime.text.MIMEText after initial assigment silently doesn\'t change them.'
updated_at = <Date 2019-03-15.15:10:43.374>
user = 'https://github.com/ukaszKaptur'

bugs.python.org fields:

activity = <Date 2019-03-15.15:10:43.374>
actor = 'Fotoblysk'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-15.15:10:43.375>
closer = 'Fotoblysk'
components = ['email']
creation = <Date 2019-03-15.13:15:00.499>
creator = 'Fotoblysk'
dependencies = []
files = []
hgrepos = []
issue_num = 36303
keywords = []
message_count = 4.0
messages = ['337986', '337988', '337989', '337997']
nosy_count = 4.0
nosy_names = ['barry', 'r.david.murray', 'remi.lapeyre', 'Fotoblysk']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36303'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[ukaszKaptur]

Script reproducing this behavior:

from email.mime.text import MIMEText
msg = MIMEText('<div/>', 'html')
msg["To"] = "this.email.shouldnt.be.printed@nokia.com"
msg["To"] = "valid.email@nokia.com"
print(msg["To"])

stdout:

this.email.shouldnt.be.printed@nokia.com

[remilapeyre]

I think this is the expected behavior, see https://docs.python.org/3.4/library/email.message.html#email.message.Message.\_\_getitem__ for details:

Note that if the named field appears more than once in the message’s headers, exactly which of those field values will be returned is undefined. Use the get_all() method to get the values of all the extant named headers.

This has been already discussed elsewhere, it may not be the best API but for compatibility reasons, we cannot change it anyway.

[remilapeyre]

Here's how you can rewrite your code so it is more explicit:

Python 3.7.2 (default, Feb 12 2019, 08:15:36)
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from email.mime.text import MIMEText
>>> msg = MIMEText('<div/>', 'html')
>>> msg.add_header('To', "this.email.shouldnt.be.printed@nokia.com")
>>> msg.add_header('To', "valid.email@nokia.com")
>>> print(msg.get_all('To'))
['this.email.shouldnt.be.printed@nokia.com', 'valid.email@nokia.com']

[ukaszKaptur]

Ok, so this is not a bug, I must missed this information in documentation.

In my defense this usage of __getitem__ and __setitem__ feels a little bit counter-intuitive.

Sorry bothering.