Python Idioms [pdf]

#1

Post by benn_88 » Thu, Jan 30, 2014, 4:18 PM UTC

Python Idioms [pdf]
safehammad.com

Re: Python Idioms [pdf]

#2

Post by [deleted] » Thu, Jan 30, 2014, 5:04 PM UTC

[deleted]

Re: Python Idioms [pdf]

#3

Post by csense » Thu, Jan 30, 2014, 5:06 PM UTC

I disagree with promoting try / catch. Exceptions like ValueError can really happen almost anywhere, so it is usually better to sanitize your inputs.

E.g. something like:

    try: 
        something = myfunc(d['x'])
    except ValueError:
        something = None

The programmer's intent is probably to only catch errors in the key lookup d['x'], but if there is some bug in the implementation of myfunc() or any of the functions called by myfunc() which causes a ValueError to unintentionally be raised, it will be caught by the except.

For dictionary lookups specifically, get() is usually preferable:

    something = d.get('x')
    if something is not None:
        something = myfunc(something)

Or if dictionary may potentially contain None values:

    if 'x' in d:
        something = myfunc(d['x'])

Re: Python Idioms [pdf]

#4

Post by simon_weber » Thu, Jan 30, 2014, 5:07 PM UTC

> 9. Create dict from keys and values using zip

In 2.7+, I'd recommend a dictionary comprehension instead.

Re: Python Idioms [pdf]

#5

Post by Redoubts » Thu, Jan 30, 2014, 5:10 PM UTC

I don't understand the truth table on slide 9 for " - vs None " and "__nonzero__ (2.x) " __bool__ (3.x) vs __nonzero__ (2.x) " __bool__ (3.x) "

Re: Python Idioms [pdf]

#6

Post by middleclick » Thu, Jan 30, 2014, 5:12 PM UTC

csense wrote: post #3

I disagree with promoting try / catch. Exceptions like ValueError can really happen almost anywhere, so it is usually better to sanitize your inputs. E.g. something like: try: something = myfunc(d['x']) except ValueError: something = None The programmer's intent is probably to only catch errors in the key lookup d['x'], but if there is some bug in the implementation of myfunc() or any of the functions called by myfun…

While not disagreeing with your case, there are some cases where I try to follow this rule because in most of those cases, I would expect that there is no exception and if there is, then catch it.

  try:
      open(FILE)
  except IOError:
      it failed

Re: Python Idioms [pdf]

#7

Post by udev » Thu, Jan 30, 2014, 5:14 PM UTC

One of the handy things not mentioned:

   with open("x.txt") as f:
       data = f.read()
       # do something with data

Re: Python Idioms [pdf]

#8

Post by hyperbovine » Thu, Jan 30, 2014, 5:18 PM UTC

His last slide could be written more idiomatically as

    ''.join('Thanks!')

Re: Python Idioms [pdf]

#9

Post by eliteraspberrie » Thu, Jan 30, 2014, 5:20 PM UTC

csense wrote: post #3

I disagree with promoting try / catch. Exceptions like ValueError can really happen almost anywhere, so it is usually better to sanitize your inputs. E.g. something like: try: something = myfunc(d['x']) except ValueError: something = None The programmer's intent is probably to only catch errors in the key lookup d['x'], but if there is some bug in the implementation of myfunc() or any of the functions called by myfun…

Also note the second argument to the get method specifies the default value to return in case of a KeyError:

    something = d.get('x', default)

Re: Python Idioms [pdf]

#10

Post by drakeandrews » Thu, Jan 30, 2014, 5:22 PM UTC

csense wrote: post #3

I disagree with promoting try / catch. Exceptions like ValueError can really happen almost anywhere, so it is usually better to sanitize your inputs. E.g. something like: try: something = myfunc(d['x']) except ValueError: something = None The programmer's intent is probably to only catch errors in the key lookup d['x'], but if there is some bug in the implementation of myfunc() or any of the functions called by myfun…

That example would surely be better as:

    something = myfunc(d['x']) if 'x' in d else None