How to Think Like a Computer Scientist - Learning with Python, 2008a

228 Debugging
A.3.2
I’ve got a big hairy expression and it doesn’t do what
Iexpect.
Writing complex expressions is fine as long as they are readable, but they can be
hard to debug. It is often a good idea to break a complex expression into a series
of assignments to temporary variables.
For example:
self.hands[i].addCard (self.hands[self.findNeighbor(i)].popCard())
This can be rewritten as:
neighbor = self.findNeighbor (i)
pickedCard = self.hands[neighbor].popCard()
self.hands[i].addCard (pickedCard)
The explicit version is easier to read because the variable names provide additional
documentation, and it is easier to debug because you can check the types of the
intermediate variables and display their values.
Another problem that can occur with big expressions is that the order of evaluation
may not be what you expect. For example, if you are translating the expression
into Python, you might write:
x
2π
y = x / 2 * math.pi
That is not correct because multiplication and division have the same precedence
and are evaluated from left to right. So this expression computes xπ/2.
A good way to debug expressions is to add parentheses to make the order of
evaluation explicit:
y = x / (2 * math.pi)
Whenever you are not sure of the order of evaluation, use parentheses. Not only
will the program be correct (in the sense of doing what you intended), it will also
be more readable for other people who haven’t memorized the rules of precedence.
A.3.3
I’ve got a function or method that doesn’t return
what I expect.
Ifyouhaveareturn statement with a complex expression, you don’t have a
chance to print the return value before returning. Again, you can use a temporary
variable. For example, instead of: