October 16th, 2009

print isn't even vaguely thread safe

Thanks to everyone who provided suggestions for my problems with print yesterday. It turns out that print isn't even vaguely threadsafe, and that the rather surprising behavior when there's a collision is for there to be duplicate output bytes. So if you ever see duplicate print lines, you know what to guess it is now.

My quick hack for getting around the problem is to make a module called rprint which looks like this (I'm using python 3):
from threading import Lock

mylock = Lock()
p = print

def print(*a, **b):
	with mylock:
		p(*a, **b)
Then from every module where I have a print I say:
from rprint import print
I don't know if this is an elegant solution or an ugly hack, but hey it works.

(no subject)

This post unintentionally demonstrates that functional-style Python is ugly and bad.

Let us start at the top. At the beginning, it has some sample code which defines a multiple() function, which could be trivially inlined, resulting in code which looks like this (I'm doing all examples in Python 3):
print(sum(x for x in range(1, 1000+1) if x % 3 == 0 or x % 5 == 0))

There's no reason whatsoever to expand that out. This should be an early indication that maybe the code samples here aren't the greatest.

Moving on, there's getting the sum of all fibonacci numbers less than 4 million. This is done in the example using itertools and yield, resulting in a fair amount of ugly code. Here's how a sane person does it:
def fibsum():
	a, b, c = 0, 1, 1
	total = 0
	while c < 4000000:
		total += c
		a, b, c = b, c, b + c
	return total

Now that's much more readable, flexible, and maintainable.

Finally, there's the problem of finding the largest palindrome which is the product of two three digit numbers. Here's my solution, which contains less code, is much more readable, and oh yeah, I threw in an optimization to make it return almost instantly instead of having to crunch for a second:
def bigpalindrome():
	best = 0
	for i in range(999, 0, -1):
		if i * 999 < best:
			return best
		for j in range(999, i-1, -1):
			x = list(str(i*j))
			x.reverse()
			if int(''.join(x)) == i*j:
				best = i*j

Much better. I think these examples do a good job of exploding the idea that the functional style of programming is clearly better and the appropriate first thing to teach people. Obviously some people are being driven to write horribly contorted and ugly code because they started in a functional language when they switch a more, ahem, mainstream one.