Anti-Patterns in Python Programming

Post by pyfish » Wed, Jul 09, 2014, 7:40 PM UTC

LyndsySimon wrote: post #117

shadowmint wrote: post #80

Mmm... you should always use 'if x is not None:' imo. It's very common for libraries to make values evaluate to False, and very easy to get bugs if you just lazily test with 'if x'. Sqlalchemy springs to mind immediately as one of the common ones where using any() and if x: is a reeeeeallly bad idea; there are plenty of others. I'm pretty skpetical about modifying your coding behavior based on what libraries you happ…

It depends. If you're checking to see if that value is None, then yes - you should check that. If you're merely checking if the value is truthy, then using "if x:" is completely legitimate.

Post by zo1 » Wed, Jul 09, 2014, 7:46 PM UTC

dagw wrote: post #46

moretti wrote: post #35

I always struggle to understand why a list comprehension alist = [foo(word) for word in words] is considered more Pythonic than map alist = map(foo, words)

List comprehensions are more flexible and easier to read in the non-trivial case. Sure in the trivial case you show a map might be considered neater, but just adding a filter is enough to make the list comprehension more readable in my mind. Python's lambda syntax also makes using maps and filters quite ugly. Compare: alist = [x**2 for x in mylist if x%3==0] to alist = map(lambda x: x**2,filter(lambda x: x%3==0, myli…

Post by rmrfrmrf » Wed, Jul 09, 2014, 7:51 PM UTC

SEJeff wrote: post #34

tribaal wrote: post #3

The more frequent and dangerous pitfalls are, in my humble opinion: - Bare except: statements (that catches everything , even Ctrl-C) - Mutables as default function/method arguments - Wildcard imports!

Couldn't agree more! One of my all time new python interview questions gets a surprisingly large number of developers. Given a function like: def append_one(l=[]): l.append(1) return l What does this return each time? >>> append_one() >>> append_one() >>> append_one()

Post by colcarroll » Wed, Jul 09, 2014, 8:01 PM UTC

SEJeff wrote: post #82

bkeroack wrote: post #21

Earlier quoted context omitted.

Depends on how you want to program: imperative vs functional. Personally I think list comprehensions are the most beautiful part of Python, though sometimes I use map() when I'm trying to be explicitly functional (I realize it's allegedly slower, etc). Generally I think list comprehensions are cleaner and allow you to write purer functions with fewer mutable variables. I disagree that deeply nested for loops are nece…

Map is not allegedly slower, it is demonstrably slower. $ python -mtimeit -s'nums=range(10)' 'map(lambda i: i + 3, nums)' 1000000 loops, best of 3: 1.61 usec per loop $ python -mtimeit -s'nums=range(10)' '[i + 3 for i in nums]' 1000000 loops, best of 3: 0.722 usec per loop Function calls have overhead in python, list comprehensions are implemented knowing this fact and avoiding it so the heavy lifting ultimately happ…

Post by kyllo » Wed, Jul 09, 2014, 8:06 PM UTC

zo1 wrote: post #128

trevor-e wrote: post #50

Earlier quoted context omitted.

I'm not a Python expert, but iirc from various blog posts the "l" variable does not get reset between function calls which will cause undesired behavior. So calling the function 3 times without argument would produce a list of size 1,2, and 3 with the third call rather than 3 lists of size 1. Can any Python guru's confirm?

The expression presented in the parameter list is only evaluated once, and that is when the method is defined. The confusion is that people assume the expression is evaluated every time the method is called.