open_resource() does not allow mode='rt'

[goodmami]

In Python 3, mode='rt' is synonymous with mode='r' but Flask disallows it thinking it is not a valid mode for reading. While this is true in Python 2, Flask could (at least) provide a more meaningful error message, if not just allow it.

A simple solution might change these lines in flask/helpers.py:

    def open_resource(self, resource, mode='rb'):
        """..."""  # truncated for example
        if mode not in ('r', 'rb'):
            raise ValueError('Resources can only be opened for reading')
        return open(os.path.join(self.root_path, resource), mode)

...to...

    def open_resource(self, resource, mode='rb'):
        """..."""
        if mode == 'rt':
            mode = 'r'
        if mode not in ('r', 'rb'):
            raise ValueError('Resources can only be opened for reading')
        return open(os.path.join(self.root_path, resource), mode)

A heavier solution might use io.open() (and also allow encoding, but see #1740):

    def open_resource(self, resource, mode='rb', encoding=None):
        """..."""
        if mode not in ('r', 'rb', 'rt'):
            raise ValueError('Resources can only be opened for reading')
        return io.open(os.path.join(self.root_path, resource), mode=mode, encoding=encoding)

Expected Behavior

Calling open_resource() with mode='rt' should open a file in text mode (at least in Python 3).

from flask import Flask

app = Flask(__name__)
with app.open_resource('README.md', mode='rt') as f:
    print(f.read())

Actual Behavior

A misleading exception is raised.

Traceback (most recent call last):
  File "mwe.py", line 5, in <module>
    with app.open_resource('README.md', mode='rt') as f:
  File "/home/goodmami/repos/OMW/py3env/lib/python3.6/site-packages/flask/helpers.py", line 1002, in open_resource
    raise ValueError('Resources can only be opened for reading')
ValueError: Resources can only be opened for reading

Environment

• Python version: 3.6.7
• Flask version: 1.0.2
• Werkzeug version: 0.14.1

[seawolf42]

Starting work on this now.

[icreatedanaccount]

PR Created #3174