This rule raises an issue when an invalid "mode" is provided to the `open` builtin function.

== Why is this an issue?

The `open` builtin function can open files in different modes. These modes are provided as a combination of characters. Using an invalid sequence of characters will make `open` fail with a `ValueError`.

A valid mode:

* should contain only one of the following characters: `r` (read), `w` (write), `a` (append), `x` (create).
* should contain zero or one of the following characters: `t` (text), `b` (binary).
* should contain zero or one `+` character (open for updating)

For example: `a`, `rt`, `r+` and `w+b` are valid modes.

If no `t` or `b` character is provided the mode will default to `t` (text), so specifying `r` is equivalent to `rt`. 

*Note*: __In Python 2, providing an incorrect mode may have an undefined behavior (ex: it might ignore some characters)__

== How to fix it

Make sure to provide a valid mode to the `open` builtin function.

=== Code examples

==== Noncompliant code example

In Python 3 the following program will throw a `ValueError`.

In Python 2.7.16 on MacOs, `open` will just ignore the `w` flag.

[source,python,diff-id=1,diff-type=noncompliant]
----
with open("test.txt", "aw") as f:  # Noncompliant: ValueError
    pass
----

==== Compliant solution

[source,python,diff-id=1,diff-type=compliant]
----
with open("test.txt", "a") as f:
    pass
----

=== Pitfalls

In Python 2, the character `U` (universal newlines) is available as a mode character but it cannot be combined with `a`, `w`, `x` or `+`. 

*Note*: __``++U++`` has no effect in Python 3, it is deprecated and is replaced by the parameter `newline` of the `open` function. The default value of `newline` is `None`, meaning universal newlines mode is enabled.__

== Resources

=== Documentation

* https://docs.python.org/3/library/functions.html#open[The `open` builtin function]


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Fix this invalid mode string.


=== Highlighting

The mode parameter


'''
== Comments And Links
(visible only on this page)

=== is related to: S5488

endif::env-github,rspecator-view[]