什么是区分字符串和列表的Pythonic方法？

Question

For my program I have a lot of places where an object can be either a string or a list containing strings and other similar lists. These are generally read from a JSON file. They both need to be treated differently. Right now, I am just using isinstance, but that does not feel like the most pythonic way of doing it, so does anyone have a better way of doing it?

Answer 1

No need to import modules, isinstance() , str and unicode (versions before 3 -- there's no unicode in 3!) will do the job for you.

Python 2.x:

Python 2.6.1 (r261:67515, Feb 11 2010, 00:51:29) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> isinstance(u'', (str, unicode))
True
>>> isinstance('', (str, unicode))
True
>>> isinstance([], (str, unicode))
False

>>> for value in ('snowman', u'☃ ', ['snowman', u'☃ ']):
...     print type(value)
... 
<type 'str'>
<type 'unicode'>
<type 'list'>

Python 3.x:

Python 3.2 (r32:88445, May 29 2011, 08:00:24) 
[GCC 4.2.1 (Apple Inc. build 5664)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> isinstance('☃ ', str)
True
>>> isinstance([], str)
False

>>> for value in ('snowman', '☃ ', ['snowman', '☃ ']):
...     print(type(value))
... 
<class 'str'>
<class 'str'>
<class 'list'>

From PEP008 :

Object type comparisons should always use isinstance() instead of comparing types directly.

Answer 2

Since Python3 no longer has unicode or basestring , in this case ( where you are expecting either a list or a string) it's better to test against list

if isinstance(thing, list):
    # treat as list
else:
    # treat as str/unicode

as that is compatible with both Python2 and Python3

Answer 3

Using isinstance :

On Python>=2.3 a string may be a str or unicode type. To check both cases:

if isinstance(a,basestring): # same as isinstance(obj, (str, unicode))
   print "Object is a string"

From Python 3 only one string type exists, so instead of basestring you should use str :

if isinstance(a,str):
   print "Object is a string"

Answer 4

Another method, using the practice of "It's better to ask forgiveness than permission," duck-typing being generally preferred in Python, you could try to do what you want first, eg:

try:
    value = v.split(',')[0]
except AttributeError:  # 'list' objects have no split() method
    value = v[0]

Answer 5

As I like to keep things simple, here is the shortest form that is compatible with both 2.x and 3.x:

# trick for py2/3 compatibility
if 'basestring' not in globals():
   basestring = str

v = "xx"

if isinstance(v, basestring):
   print("is string")

Answer 6

You can use types module:

import types
if type(ls) == types.ListType:
    #your code for list objects here