For a long time, people have been requesting a way to write

conditional expressions, expressions that return value A or value B

depending on whether a Boolean value is true or false. A conditional

expression lets you write a single assignment statement that has the

same effect as the following:

\begin{verbatim}

\end{verbatim}

There have been endless tedious discussions of syntax on both

python-dev and comp.lang.python, and even a vote that found the

majority of voters wanted some way to write conditional expressions,

but there was no syntax that was clearly preferred by a majority.

Candidates include C's \code{cond ? true_v : false_v},

\code{if cond then true_v else false_v}, and 16 other variations.

GvR eventually chose a surprising syntax:

\begin{verbatim}

\end{verbatim}

Evaluation is still lazy as in existing Boolean expression, so the

evaluation jumps around a bit. The \var{condition} expression is

evaluated first, and the \var{true_value} expression is evaluated only

if the condition was true. Similarly, the \var{false_value}

expression is only evaluated when the condition is false.

This syntax may seem strange and backwards; why does the condition go

in the \emph{middle} of the expression, and not in the front as in C's

\code{c ? x : y}? The decision was checked by applying the new syntax

to the modules in the standard library and seeing how the resulting

code read. In many cases where a conditional expression is used, one

value seems to be the 'common case' and one value is an 'exceptional

case', used only on rarer occasions when the condition isn't met. The

conditional syntax makes this pattern a bit more obvious:

\begin{verbatim}

\end{verbatim}

I read the above statement as meaning ``here \var{contents} is

\var{doc} is empty, in which special case an empty string is returned.''

I doubt I will use conditional expressions very often where there

isn't a clear common and uncommon case.

There was some discussion of whether the language should require

surrounding conditional expressions with parentheses. The decision

was made to \emph{not} require parentheses in the Python language's

grammar, but as a matter of style I think you should always use them.

Consider these two statements:

\begin{verbatim}

\end{verbatim}

In the first version, I think a reader's eye might group the statement

into 'level = 1', 'if logging', 'else 0', and think that the condition

decides whether the assignment to \var{level} is performed. The

second version reads better, in my opinion, because it makes it clear

that the assignment is always performed and the choice is being made

between two values.

Another reason for including the brackets: a few odd combinations of

list comprehensions and lambdas could look like incorrect conditional

expressions. See \pep{308} for some examples. If you put parentheses

around your conditional expressions, you won't run into this case.

\begin{seealso}

\seepep{308}{Conditional Expressions}{PEP written by

Guido van Rossum and Raymond D. Hettinger; implemented by Thomas

Wouters.}

\end{seealso}