Programming safely with Python
People often say that Python is an easy language, but like other easy languages it is often harder to make sure you are working in a safe manner. I'm not talking about security, but bug-free.
A) Assert allot:
assert something == True #This must be true because...
assert isinstance(variable, int) #This algorithm only makes sense with intergersRule of thumb: if you create code with a given assumption, use an assert and place a comment to document the assert. Asserts will make your code slower, but if everything works you can use
python -O to remove the asserts.
B) Keep your scope clean:
This is why I started this post, if you have a loop your loop variable will be in the loop declaration scope and not inside the loop. It could come in handy if you want to see how far a loop got, but there are also problems. Each assignment in Python is also an implicit definition, which could make a typo problematic:
bestValues = "abcd"
for bestValue in bestValues:
print bestValue
bestValeu = "c"
for i in bestValue:
print iThe typo will not be noticed as the first loop will define bestValue already effectively silencing the faulty assignment. The result is getting "d" on the screen instead of "c". Another example shows you that you may get a problem:
#sockets is a list of sockets
for s in sockets:
s.send("ok, thank you")
del sockets #That's it, close all the sockets
#Reconnect for a new transaction
sockets = [socket("192.168.1.%i" % i) for i in range(255)]The
del sockets line will not work as expected because the last socket in the list is still alive as the variable "s".
One way to keep this from happening is using del after every loop:
for a in values:
print a
del a
b = [i/2.0 for i in xrange(10)]
del iThink that solves it? Well, no, not really. Here is a code example that shows the error:
def formatit(a):
s = "see:"
for i in a:
s += " " + str(i)
del i
return sThis code works (and is ugly) but the
del i will raise an error when you call "formatit([])". It is comprehensible result as the loop is not run, so why should i be defined in the first place? But this is where a habit of using del i will not be a good thing, because in this context del i is effectively assert len(a); del i (which can be nice if you actually want that).
This is where I'm not sure what best practice to choose. I think I'm going to have to go with the following solution to keep safe without thinking to much. A good habbit may be: use only a single loop in every scope, if you decide to use multiple then place each in a separate scope using if True:
if True:
for i in something:
print iIt is safer and easier then using del yourself, and probably optimized out at some point in the future.