BUG: various bug fixes for DataFrame/Series construction
0 and 1 len ndarrays not inferring dtype correctly
datetimes that are single objects not inferring dtype
mixed datetimes and objects (GH #2751), casting datetimes to object
timedelta64 creation on series subtraction (of datetime64[ns])
astype on datetimes to object are now handled (as well as NaT conversions to np.nan)

astype conversion

```
In [4]: s = pd.Series([datetime.datetime(2001, 1, 2, 0, 0) for i in range(3)])

In [5]: s.dtype
Out[5]: dtype('datetime64[ns]')

In [6]: s[1] = np.nan

In [7]: s
Out[7]: 
0   2001-01-02 00:00:00
1                   NaT
2   2001-01-02 00:00:00

In [8]: s.dtype
Out[8]: dtype('datetime64[ns]')

In [9]: s = s.astype('O')

In [10]: s
Out[10]: 
0    2001-01-02 00:00:00
1                    NaN
2    2001-01-02 00:00:00

In [11]: s.dtype
Out[11]: dtype('object')
```

construction with datetimes

```
In [7]: pd.DataFrame({'A' : 1, 'B' : 'foo', 'C' : 'bar', 
    'D' : pd.Timestamp("20010101"),
    'E' : datetime.datetime(2001,1,2,0,0) }, index=np.arange(5)).get_dtype_counts()
Out[7]: 
datetime64[ns]    2
int64             1
object            2

In [16]: pd.DataFrame({'a':[1,2,4,7], 'b':[1.2, 2.3, 5.1, 6.3], 
    'c':list('abcd'), 
    'd':[datetime.datetime(2000,1,1) for i in range(4)] }).get_dtype_counts()
Out[16]: 
datetime64[ns]    1
float64           1
int64             1
object            1
```

1 len  ndarrays

```
In [16]: pd.DataFrame({'a': 1., 'b': 2, 'c': 'foo', 'float64' : np.array(1.,dtype='float64'),
    'int64' : np.array(1,dtype='int64')}, index=np.arange(10)).get_dtype_counts()
Out[16]: 
float64    2
int64      2
object     1
```

Thanks jeff
 fyi: this combined with #2708 is causing `test_constructor_with_datetimes` and `test_get_numeric_data` to fail on 32-bit, because it is assuming that dtypes default to int64
 just saw that
I can do a PR to fix the test a bit later or
u r welcome to do if u want
 actually, here's what i have:

``` python
        intname = np.dtype(np.int_).name
        floatname = np.dtype(np.float_).name
        datetime64name = np.dtype('M8[ns]').name
        objectname = np.dtype(np.object_).name

        # single item
        df = DataFrame({'A' : 1, 'B' : 'foo', 'C' : 'bar', 'D' : Timestamp("20010101"), 'E' : datetime(2001,1,2,0,0) },
                       index=np.arange(10))
        result = df.get_dtype_counts()
        expected = Series({intname: 1, datetime64name: 2, objectname : 2})
        assert_series_equal(result, expected)
```

that's working fine

but this is not:

``` python
        # GH #2751 (construction with no index specified)
        df = DataFrame({'a':[1,2,4,7], 'b':[1.2, 2.3, 5.1, 6.3], 'c':list('abcd'), 'd':[datetime(2000,1,1) for i in range(4)] })
        result = df.get_dtype_counts()
        expected = Series({intname: 1, floatname : 1, datetime64name: 1, objectname : 1})
        assert_series_equal(result, expected)
```

because the integer list is being promoted to int64, even on 32-bit. is that intended?

see commit 9a047679c2e6f2064fb4b656a2461cceba7df679 (b05b97986f779cdd9007f281c4255c0d31ab263c below, if it's still showing, is out of date)
 ``` python
In [29]: p.DataFrame([1,2]).dtypes
Out[29]: 
0    int32
Dtype: object

In [30]: p.DataFrame({'a': [1,2]}).dtypes
Out[30]: 
a    int64
Dtype: object
```
 this does look inconsistent
will see of I can track it down

is your example on 32-bit?

(default for ints should be np.int_)
- platform dependent (should not be upcasted)

On Feb 10, 2013, at 6:13 PM, stephenwlin notifications@github.com wrote:

> In [29]: p.DataFrame([1,2]).dtypes
> Out[29]: 
> 0    int32
> Dtype: object
> 
> In [30]: p.DataFrame({'a': [1,2]}).dtypes
> Out[30]: 
> a    int64
> Dtype: object
> —
> Reply to this email directly or view it on GitHub..
 yeah, 32-bit
 ok, just fyi to save you time, I've narrowed it down to `lib.maybe_convert_objects`, called by way of `series._sanitize_array`

here's the relevant lines, with some print statements added (and what they print when calling `p.DataFrame.from_dict({'a': [1,2]})`)

``` python
            print data, np.asarray(data).dtype 
            # "[1, 2] int32"
            subarr = lib.list_to_object_array(data)
            print subarr, subarr.dtype
            # "[1 2] object"
            print [np.asarray(x).dtype for x in subarr]
            # "[dtype('int32'), dtype('int32')]"
            subarr = lib.maybe_convert_objects(subarr)
            print subarr, subarr.dtype
            # "[1 2] int64"
            subarr = com._possibly_cast_to_datetime(subarr, dtype)
            print subarr, subarr.dtype
            # "[1 2] int64"
```

i can look at it further if you want, but since you've been working on this I'm guessing you're more familiar with it and less likely to break something else with a fix (if this needs to be fixed)
 hmm
didn't do anything here
but if I recall maybe_convert_objects I think DOES up convert (so will have to take that out - and hope doesn't break anything else)

On Feb 10, 2013, at 6:40 PM, stephenwlin notifications@github.com wrote:

> ok, just fyi to save you time, I've narrowed it down to lib.maybe_convert_objects, called by way of series._sanitize_array
> 
> here's the relevant lines, with some print statements added (and what they print when calling p.DataFrame.from_dict({'a': [1,2]}))
> 
> ```
>         print data, np.asarray(data).dtype # prints "[1, 2] int32"
>         subarr = lib.list_to_object_array(data)
>         print subarr, subarr.dtype # prints "[1 2] object"
>         subarr = lib.maybe_convert_objects(subarr)
>         print subarr, subarr.dtype # prints "[1 2] int64"
>         subarr = com._possibly_cast_to_datetime(subarr, dtype)
>         print subarr, subarr.dtype # prints "[1 2] int64"
> ```
> 
> ``
> 
> i can look at it further if you want, but since you've been working on this I'm guessing you're more familiar with this and less likely to break something else with a fix (if this needs to be fixed)
> —
> Reply to this email directly or view it on GitHub..
 ok made a PR to just fix the build for now (leaving that particular test referencing int64 instead of platform int)
