enpassant 0.1

En passant (in passing) assignment for Python

Provides a general-purpose en passant assignment / naming mechanism for Python.

Many languages support en passant (in passing) assignment, like so:

if result = expensive_request():
    print result.report()

Python does not. This leads to more code lines and (in my opinion) less visual clarity:

result = expensive_request()
if result:
    print result.report()

Or worse, in the case of looping structures:

result = expensive_request()
while result:
    print result.report()
    result = expensive_request()

It doesn’t look so bad here, in a highly distilled example, but in real programs, the called function often has parameters to be managed, and the surrounding code is invariably longer and more compliated. The more complicated the surrounding computations and requests, the simpler the comparision itself should be.

I hope that Python will eventually provide a concise way of handling this, such as:

while expensive_request() as result:
    print result.report()

But in the meanwhile, this module provides a workaround.

Usage

from enpassant import *
result = Passer()

while result / expensive_request():
    print result.value.report()

result is merely a proxy object that, when it encounters the division operator, returns the denominator. That is, result / whatever == whatever. But it also remembers the denominator value. Then, whenever you want the result value provided (presumably, later in the body of your loop or conditional), simply ask for result.value. Easy peasy!

Details and Options

enpassant “assignment” is transparent to conditional expressions, because the value of the expression is always the value of the denominator. But Passers are also guaranteed to have an boolean value identical to that of the value they contain, should you wish to use them in subsequent tests.

If you prefer the look of the less-than (<) or less-than-or-equal (<=) operators as indicators that result takes the value of the following value, they are supported as aliases of the division operation (/). Thus, the following are identical:

if result / expensive_request():
    print "yes"

if result < expensive_request():
    print "yes"

if result <= expensive_request():
    print "yes"

It’s a matter of preference which seems most logical, appropriate, and expressive. Note, however, that the operation usually known as division (/) has a much higher precedence (i.e. tighter binding to its operands) than the typical comparison operations (< and <=). If used with a more complex expressions, either know your prececence or use parenthesis to disambiguate!

Notes

• En passant assignment / naming is discussed in Issue1714448 and PEP 379, which have been rejected and withdrawn, respectively. But they are years gone by. I hope the idea will be productively reconsidered in the future.

• Automated multi-version testing is managed with the wonderful pytest and tox. enpassant is successfully packaged for, and tested against, all late-model verions of Python: 2.6, 2.7, 3.2, and 3.3, as well as PyPy 1.9 (based on 2.7.2).

• The simplere package provides more extensive en passant handling in the important, common case of regular expression searches.

• The author, Jonathan Eunice or @jeunice on Twitter welcomes your comments and suggestions.

Installation

To install the latest version:

pip install -U enpassant

To easy_install under a specific Python version (3.3 in this example):

python3.3 -m easy_install --upgrade enpassant

(You may need to prefix these with “sudo “ to authorize installation.)