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!

a return to yield

I remember when, almost a decade ago, I was first discovering generators. It was a heady time, and I saw applications everywhere.

def fib_gen():
    x, y = 1, 1
    while x < 100:
       x, y = y, x + y
       yield x
    return

I also remember the first time I tried to mix a return value into my generator.

def fib_gen():
    x, y = 1, 1
    while x < 100:
       x, y = y, x + y
       yield x
    return True

Imagine my surprise, as I'm sure countless others experienced as well:

SyntaxError: 'return' with argument inside generator

A rare compile-time error! Only the decorative, bare return is allowed in generators, where they serve to raise StopIteration.

Now, imagine my surprise, so many years later when I import that same code in Python 3.


...

Nothing! No error. So what happened?

Turns out that the coroutine and asyncio machinery of Python 3 has repurposed this old impossibility.

If we manually iterate to skip over our yield:

fib_iter = fib_gen()                                                                                                                                                                                                    
for i in range(11):                                                                                                                 
    next(fib_iter)                                                                                                                  
next(fib_iter)

We see what's really happening with our return:

Traceback (most recent call last):
  File "fib_gen.py", line 13, in <module>
    next(fib_iter)
StopIteration: True

That's right, returns in generators now raise StopIteration with a single argument of the return value.

Most of the time you won't see this. StopIterations are automatically consumed and handled correctly by for loops, list comprehensions, and sequence constructors (like list). But it's yet another reason to be extra careful when writing your own generators, specific to Python 3.

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