>I don't know the history, but it seems like that might be one of the reasons `x() if y()` isn't allowed in python.

Maybe I misunderstood your context, but if not, conditional expressions do exist in Python:

  $ python
  Python 2.7.12 |Anaconda 4.2.0 (32-bit)| (default, Jun 29 2016, 11:42:13) [MSC v.1500 32 bit (Intel)] on win32
  Type "help", "copyright", "credits" or "license" for more information.
  Anaconda is brought to you by Continuum Analytics.
  Please check out: http://continuum.io/thanks and   https://anaconda.org
  >>> 1 if 1 == 1 else 2
  1
  >>> 2 if 1 == 1 else 2
  2
  >>> ^Z

  $ py -3
  Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 27 2018, 04:59:51) [MSC v.1914 64 bit (AMD64)] on win32
  Type "help", "copyright", "credits" or "license" for more   information.
  >>> 1 if 1 == 1 else 2
  1
  >>> 2 if 1 == 1 else 2
  2
  >>>

Using nested conditional expressions to classify characters:

https://jugad2.blogspot.com/2017/04/using-nested-conditional...

Sorry, the code shown in the two Python interpreter sessions above, is not a good example. To make the concept of conditional expressions more clear, this example is better:

  >>> from __future__ import print_function
  >>> for i in range(3, 5):
  ...     print(i, "is", "odd" if i % 2 == 1 else "even")
  ...
  3 is odd
  4 is even

which can be shortened to this:

  >>> for i in range(3, 5):
  ...     print(i, "is", "odd" if i % 2 else "even")
  ...
  3 is odd
  4 is even

Now print() is printing 3 values: i, "is", and either "odd" or "even" based on the boolean condition.

That works in both Python 2 and 3.