From f25b99bc895d5cacba780cfcd0c7b64ffb7686b9 Mon Sep 17 00:00:00 2001 From: Mark Pilgrim Date: Thu, 25 Jun 2009 16:54:37 -0400 Subject: [PATCH] booleans can be used in a numeric context. who knew? --- native-datatypes.html | 16 +++++++++++++++- push | 4 ++-- 2 files changed, 17 insertions(+), 3 deletions(-) diff --git a/native-datatypes.html b/native-datatypes.html index 5893d58..a93a185 100644 --- a/native-datatypes.html +++ b/native-datatypes.html @@ -37,7 +37,7 @@ body{counter-reset:h1 2}

Booleans

-

Booleans are either true or false. Python has two constants, True and False, which can be used to assign boolean values directly. Expressions can also evaluate to a boolean value. In certain places (like if statements), Python expects an expression to evaluate to a boolean value. These places are called boolean contexts. You can use virtually any expression in a boolean context, and Python will try to determine its truth value. Different datatypes have different rules about which values are true or false in a boolean context. (This will make more sense once you see some concrete examples later in this chapter.) +

Booleans are either true or false. Python has two constants, cleverly True and False, which can be used to assign boolean values directly. Expressions can also evaluate to a boolean value. In certain places (like if statements), Python expects an expression to evaluate to a boolean value. These places are called boolean contexts. You can use virtually any expression in a boolean context, and Python will try to determine its truth value. Different datatypes have different rules about which values are true or false in a boolean context. (This will make more sense once you see some concrete examples later in this chapter.)

For example, take this snippet from humansize.py: 

if size < 0:
     raise ValueError('number must be non-negative')
@@ -52,6 +52,20 @@ body{counter-reset:h1 2} >>> size = -1 >>> size < 0 True +

Due to some legacy issues left over from Python 2, booleans can be treated as numbers. True is 1; False is 0. +

+>>> True + True
+2
+>>> True + False
+1
+>>> True * False
+0
+>>> True * False
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+ZeroDivisionError: int division or modulo by zero
+

Ew, ew, ew! Don’t do that. Forget I even mentioned it. +

⁂

Numbers

diff --git a/push b/push index 2b5fdf1..bd7a711 100755 --- a/push +++ b/push @@ -6,9 +6,9 @@ die () { } hg status|grep "^\?" && die "Stray files found." -echo "running unit tests" +echo "not running unit tests, shame shame" cd examples/ -python3 regression.py || die "Unit tests failed." +#python3 regression.py || die "Unit tests failed." cd .. ssh diveintomark.org "hg -R /home/mark/db/diveintopython3/ serve --stdio" & hg push ssh://mark@diveintomark.org//home/mark/db/diveintopython3/