Hidden features of Python [closed]

Answer 1

The step argument in slice operators. For example:

a = [1,2,3,4,5]
>>> a[::2]  # iterate over the whole list in 2-increments
[1,3,5]

The special case x[::-1] is a useful idiom for 'x reversed'.

>>> a[::-1]
[5,4,3,2,1]

Answer 2

Creating new types in a fully dynamic manner

>>> NewType = type("NewType", (object,), {"x": "hello"})
>>> n = NewType()
>>> n.x
"hello"

which is exactly the same as

>>> class NewType(object):
>>>     x = "hello"
>>> n = NewType()
>>> n.x
"hello"

Probably not the most useful thing, but nice to know.

Edit: Fixed name of new type, should be NewType to be the exact same thing as with class statement.

Edit: Adjusted the title to more accurately describe the feature.

Answer 3

To add more python modules (espcially 3rd party ones), most people seem to use PYTHONPATH environment variables or they add symlinks or directories in their site-packages directories. Another way, is to use *.pth files. Here's the official python doc's explanation:

"The most convenient way [to modify python's search path] is to add a path configuration file to a directory that's already on Python's path, usually to the .../site-packages/ directory. Path configuration files have an extension of .pth, and each line must contain a single path that will be appended to sys.path. (Because the new paths are appended to sys.path, modules in the added directories will not override standard modules. This means you can't use this mechanism for installing fixed versions of standard modules.)"

Answer 4

Re-raising exceptions:

# Python 2 syntax
try:
    some_operation()
except SomeError, e:
    if is_fatal(e):
        raise
    handle_nonfatal(e)

# Python 3 syntax
try:
    some_operation()
except SomeError as e:
    if is_fatal(e):
        raise
    handle_nonfatal(e)

The 'raise' statement with no arguments inside an error handler tells Python to re-raise the exception with the original traceback intact, allowing you to say "oh, sorry, sorry, I didn't mean to catch that, sorry, sorry."

If you wish to print, store or fiddle with the original traceback, you can get it with sys.exc_info(), and printing it like Python would is done with the 'traceback' module.

Answer 5

If you don't like using whitespace to denote scopes, you can use the C-style {} by issuing:

from __future__ import braces

Answer 6

From 2.5 onwards dicts have a special method __missing__ that is invoked for missing items:

>>> class MyDict(dict):
...  def __missing__(self, key):
...   self[key] = rv = []
...   return rv
... 
>>> m = MyDict()
>>> m["foo"].append(1)
>>> m["foo"].append(2)
>>> dict(m)
{'foo': [1, 2]}

There is also a dict subclass in collections called defaultdict that does pretty much the same but calls a function without arguments for not existing items:

>>> from collections import defaultdict
>>> m = defaultdict(list)
>>> m["foo"].append(1)
>>> m["foo"].append(2)
>>> dict(m)
{'foo': [1, 2]}

I recommend converting such dicts to regular dicts before passing them to functions that don't expect such subclasses. A lot of code uses d[a_key] and catches KeyErrors to check if an item exists which would add a new item to the dict.