None on the left

A natural default, None is probably the most commonly assigned value in Python. But what happens if you move it to the left side of that equation?

In Python 2:

>>> None = 2
  File "<stdin>", line 1
SyntaxError: cannot assign to None

This is similar to what happens when you assign to a literal:

>>> 1 = 2
  File "<stdin>", line 1
SyntaxError: can't assign to literal

In Python 3 this walk on the wild side will get you a slightly different error:

>>> None = 1
  File "<stdin>", line 1
SyntaxError: can't assign to keyword

None has graduated from useful snowflake to full-blown keyword!

Undecoratable

Decorators are one of Python's bigger success stories, and many programmers' first experience with higher-order programming. Most practiced and prolific Python programmers will find themselves making good use of them regularly.

But every feature has its limits, and here's a new one to try on for size:

>>> @x().y()
  File "<stdin>", line 1
    @x().y()
        ^
SyntaxError: invalid syntax

That's right, decoration is not an arbitrary Python expression. It doesn't matter what x and y were, or even if they were defined. You can't follow a function call with a dot. @x() works fine, @x.y() would work fine, too. But @x().y(), that's only for mad Pythonists who would take things TOO FAR.

Decorator invocations, defined at the top of the Python grammar, can only be followed by class definitions and function definitions.

Well, now we know, and now we can all say we've been there

-- Mahmoud
http://sedimental.org/
https://github.com/mahmoud
https://twitter.com/mhashemi

Loopy references

This may not come as a surprise to experienced Python programmers, but objects in Python can reference or "contain" themselves and/or their parent objects.

A trivial example, the self-containing list:

>>> a = []
>>> a.append(a)
>>> a
[[...]]

Kind of a cool repr, at least. How else could you flatly represent this infinitely nested object? What happens when you take two of these and smash them together with an equals comparator?

>>> b = []
>>> b.append(b)
>>> a == b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: maximum recursion depth exceeded in cmp

Like two mirrors pointed at one another, they compare back and forth forever, or at least for...

>>> sys.getrecursionlimit()
1000 

... loops.