Python: Even a Feature That You Do Not Use Can Bite You

Weird article title. This is more neat than it is alarming.

Local variable type hints are a major reason I refuse to use python versions before 3.6. What is this guy's problem?

non-clickbait title: Python: Mistyping a character in your code can bite you.

Any IDE or linter worth its salt would catch this for you.

Understanding the basic syntax for assignment isn’t a high bar to clear. While it’s maybe not ideal to use colons for both the “assignment-like” separator between keys and values inside dictionaries and the annotation of types, introducing any new syntax features into a language that’s been around for decades is never without compromise. Colons are also used at the end of block statements and in index ranges and nobody complains.

In this case you hit the most benign sort of change: Code that was previously syntactically invalid is now syntactically valid but has silly semantics. There’s never going to be a way to avoid this with full generality, and your code never worked — type annotations just delayed your runtime error by one line.

When annotations were introduced in 3.4, there was no requirement they be used to annotate types instead of something else, so any valid Python expression is allowed syntactically. In 3.6 the use of annotations for anything other than types was provisionally deprecated, and in 3.7 is fully deprecated. In 3.8, non-type usage will be disallowed as a TypeError, and the only allowable non-type annotations will be strings, (which will be interpreted as bare identifiers for backward compatibility with pre-3.7 code that uses them as forward references). At this point your code will revert to raising an exception on exactly the same line it used to.

The level of assumed ignorance of the reader in this article is a bit high given the subject matter.