datetime.datetime对象的len（）

Question

I am trying to get the length of various types of data in python:

 if type(rowoutClipFC[fieldnum]) in (int, float, datetime.datetime):
       lenrowoutClipFC = len(str(rowoutClipFC[fieldnum]))
 else:
       lenrowoutClipFC = len(rowoutClipFC[fieldnum])

The types I have come across so far are int(), float(), str(), str() with ascii characters, datetime.datetime.

I need to convert the int(), float(), datetime.datetime objects to str() so I can get length, whereas I do not need to do this with the str() and str() with ascii. For some reason, datetime.datetime does not pass the smell test of the first if statement, so it errors. I'm not sure why it isn't....

the error returns:

lenrowoutClipFC = len(rowoutClipFC[fieldnum])
TypeError: object of type 'datetime.datetime' has no len()

So, it's moving onto the else statement which is not what I was expecting...

EDIT:

What I am expecting is this:

for row in arcpy.SearchCursor("Strategic_Land_Resource_Planning_Area"):
    type(row.APPROVAL_DATE)

This returns datetime.datetime

The field in the table I am pulling the data from is a 'date' field. So, I guess it returns a datetime.datetime type...even though someone mentioned that there is no such thing as a datetime.datetime type?

So, I further test:

import datetime
for row in arcpy.SearchCursor("Strategic_Land_Resource_Planning_Area"):
    if type(row.APPROVAL_DATE) == datetime.datetime:
         print 'Yes"

This returns Yes

So, I am not sure why my script at the top doesn't satisfy the if condition....

I tries using isinstance with the same reults. It doesn't satisfy in the if statement.

Answer 1

It's because the type of datetime.datetime isn't 'datetime', it's 'type'. That's because datetime.datetime is the class, not an instance of the class.

Eg

>>> import datetime

>>> type(datetime.datetime)
<type 'type'>

>>> type(datetime.datetime.now())
<type 'datetime.datetime'>

>>> type(int)
<type 'type'>
>>>

The docs recommend if you're testing the type of something, to use isinstance() rather than type() (ref https://docs.python.org/2/library/functions.html#type )

Try this instead:

field = rowoutClipFC[fieldnum]
if type(field) in (int, float) or isinstance(field, datetime.datetime):
    lenrowoutClipFC = len(str(field))
else:
    lenrowoutClipFC = len(field)

Even simpler, you could just do this:

field = rowoutClipFC[fieldnum]
lenrowoutClipFC = len(str(field))

Answer 2

To explain this a little

I need to convert the int(), float(), datetime.datetime objects to str() so I can get length, whereas I do not need to do this with the str() and str() with ascii.

In Python there are a number of built in sequence types, including strings, unicode strings, lists, tuples etc. You can pass any of these types to len() and a integer will be returned, as there is a protocol in C Python at least which expects these objects to support this behaviour.

>>> len("hello")
5
>>> len(["foo", "bar"]
2

Many other objects can also be passed to len() , including built-ins types like dictionaries and custom objects which implement a __len__() magic method.

>>> len({"foo":"bar"}
1

Some objects which are not sequences / containers do not support this however, such as integer and float types like you mentioned. If you try and pass such objects to len() , a TypeError exception is raised.

>>> len(5)
TypeError: object of type 'int' has no len()

Same goes for datetime objects as you've found out

>>> from datetime import datetime
>>> len(datetime.today())
TypeError: object of type 'datetime.datetime' has no len()

And this makes sense - len() should indicate the number of items in an object. What would you expect a datetime object to return?

So you can convert the datetime object into a string and count it's characters, but that doesn't really tell you much.

>>> datetime.datetime.today()
'2014-04-12 00:36:03.829979'
>>> len(str(datetime.today()))
26

Ultimately you shouldn't be trying to call len() on a datetime object at all - so you should rethink what you are trying to achieve with this code.

Answer 3

Can you try this:

import datetime
for row in arcpy.SearchCursor("Strategic_Land_Resource_Planning_Area"):
    if isinstance(row.APPROVAL_DATE), datetime.datetime):
         print "Yes"
    else:
         print "No"

Does that print "Yes"?