From 88434812673f9551ce6724ef6d56aefe1330127d Mon Sep 17 00:00:00 2001 From: Mark Pilgrim Date: Wed, 15 Jul 2009 10:08:37 -0400 Subject: [PATCH] added example of human-readable message in assert statement --- advanced-iterators.html | 23 +++++++++++++---------- examples/alphametics.py | 2 +- 2 files changed, 14 insertions(+), 11 deletions(-) diff --git a/advanced-iterators.html b/advanced-iterators.html index 4b64023..990afb3 100755 --- a/advanced-iterators.html +++ b/advanced-iterators.html @@ -51,7 +51,7 @@ import itertools def solve(puzzle): words = re.findall('[A-Z]+', puzzle.upper()) unique_characters = set(join(words)) - assert len(unique_characters) <= 10 + assert len(unique_characters) <= 10, 'Too many letters' first_letters = {word[0] for word in words} n = len(first_letters) sorted_characters = ''.join(first_letters) + \ @@ -161,28 +161,31 @@ if __name__ == '__main__':

Like many programming languages, Python has an assert statement. Here’s how it works. 

->>> assert 1 + 1 == 2  ①
->>> assert 1 + 1 == 3  ②
+>>> assert 1 + 1 == 2                                     ①
+>>> assert 1 + 1 == 3                                     ②
 Traceback (most recent call last):
   File "<stdin>", line 1, in 
-AssertionError
+AssertionError +>>> assert 2 + 2 == 5, "Only for very large values of 2" ③ +Traceback (most recent call last): + File "", line 1, in +AssertionError: Only for very large values of 2
  1. The assert statement is followed by any valid Python expression. In this case, the expression 1 + 1 == 2 evaluates to True, so the assert statement does nothing.
  2. However, if the Python expression evaluates to False, the assert statement will raise an AssertionError. +
  3. You can also include a human-readable message that is printed if the AssertionError is raised.

Therefore, this line of code: -

assert len(unique_characters) <= 10
+
assert len(unique_characters) <= 10, 'Too many letters'
-

…is equivalent to… +

…is equivalent to this: 

if len(unique_characters) > 10:
-    raise AssertionError
+ raise AssertionError('Too many letters') -

But a bit easier to read and write. - -

The alphametics solver uses this exact assert statement to bail out early if the puzzle contains more than ten unique letters. Since each letter is assigned a unique digit, and there are only ten digits, a puzzle with more than ten unique letters is unsolvable. +

The alphametics solver uses this exact assert statement to bail out early if the puzzle contains more than ten unique letters. Since each letter is assigned a unique digit, and there are only ten digits, a puzzle with more than ten unique letters can not possibly have a solution.

⁂ diff --git a/examples/alphametics.py b/examples/alphametics.py index 90840df..f1f4c0b 100755 --- a/examples/alphametics.py +++ b/examples/alphametics.py @@ -10,7 +10,7 @@ import itertools def solve(puzzle): words = re.findall('[A-Z]+', puzzle.upper()) unique_characters = set(''.join(words)) - assert len(unique_characters) <= 10 + assert len(unique_characters) <= 10, 'Too many letters' first_letters = {word[0] for word in words} n = len(first_letters) sorted_characters = ''.join(first_letters) + \