seanmonstar

Jan 14 2010

3 Tips When Switching to Python

If you write a lot of Javascript or PHP, there are a couple of habits you might be used to that need to change a bit when you switch over to Python.

  1. Accessing a property in a dictionary with a variable
  2. Setting properties on objects with a variable
  3. Using While with a function call

Check in first

When looping through a list or dictionary, it’s not uncommon to compare the current indexed value to a value in a different list or dictionary. However, doing that will quickly teach you that trying to access a key that doesn’t exist will raise a KeyError.

After my first KeyError, I first tried to wrap the comparison in a try block. But that can start to look unwieldly:

try:
	val = params[key]
except KeyError:
	val = None
if val:
	#...

Alternatively, Python dictionaries have a get method. Hence we do this, and it’s all pretty!

val = params.get(key, 'defaultVal')

You can just setattr

A while ago, when doing some initial Django development, I tried to naively handle submission of forms the same way I do in PHP. I loop through each key value pair in the POST dictionary, and assign it to an instance of the model I want to insert. No worries if extra information has been submitting, the model will send data that we have specifically set at fields in the class definition. However, objects in Python don’t allow item assignment like PHP or Javascript. Every object does have a personal ` dict ` that I could access, but then I can get KeyErrors as the above example shows.

Denis Otkidach showed me Python’s setattr function, which lets me do exactly what I wanted.

for key, value in POST.iteritems():    
	setattr(my_model, key, value)

While True, loop foreverrrrr

Often times, when you don’t have a predetermined length of something, you’ll use while to do your looping. A common occurrence of this is when reading in a file. You call a function, and store its return value, and as long as that value is usable, do your loopity loop stuff. However, in Python you can’t do assignment inside a condition for a control structure like while, probably because Guido likes to prevent bad practices from being possible in his language, and that is usually a bad practice unless you know what you’re doing.

There maybe a more elegant way of handling this, but i resorted to making an infinite loop that breaks on a bad condition:

while True:
	val = func()
	if val:
		pass
	else:
		break

If you write a lot of Javascript or PHP, there are a couple of habits you might be used to that need to change a bit when you switch over to Python.

  1. Accessing a property in a dictionary with a variable
  2. Setting properties on objects with a variable
  3. Using While with a function call
  • #python