Python Cookbook

Assigning and Testing with One Statement

Credit: Alex Martelli

Problem

You are transliterating C or Perl code to Python, and, to keep close to the original’s structure, you need an expression’s result to be both assigned and tested (as in if((x=foo( )) or while((x=foo( )) in such other languages).

Solution

In Python, you can’t code:

if x=foo(  ):

Assignment is a statement, so it cannot fit into an expression, which is necessary for conditions of if and while statements. Normally this isn’t a problem, as you can just structure your code around it. For example, this is quite Pythonic:

while 1:
    line = file.readline(  )
    if not line: break
    process(line)

In modern Python, this is far better, but it’s even farther from C-like idioms:

for line in file.xreadlines(  ):
    process(line)

In Python 2.2, you can be even simpler and more elegant:

for line in file:
    process(line)

But sometimes you’re transliterating C, Perl, or some other language, and you’d like your transliteration to be structurally close to the original.

One simple utility class makes this easy:

class DataHolder:
    def _ _init_ _(self, value=None):
        self.value = value
    def set(self, value):
        self.value = value
        return value
    def get(self):
        return self.value
# optional and strongly discouraged, but handy at times:
import _ _builtin_ _
_ _builtin_ _.DataHolder = DataHolder
_ _builtin_ _.data = DataHolder(  )

With the help of the DataHolder class and its data instance, you can keep your C-like code structure intact in transliteration:

while data.set(file.readline( ...