The tolist() method should do what you want. If you have a numpy array, just call tolist():

In [17]: a
Out[17]: 
array([ 0.        ,  0.14285714,  0.28571429,  0.42857143,  0.57142857,
        0.71428571,  0.85714286,  1.        ,  1.14285714,  1.28571429,
        1.42857143,  1.57142857,  1.71428571,  1.85714286,  2.        ])

In [18]: a.dtype
Out[18]: dtype('float64')

In [19]: b = a.tolist()

In [20]: b
Out[20]: 
[0.0,
 0.14285714285714285,
 0.2857142857142857,
 0.42857142857142855,
 0.5714285714285714,
 0.7142857142857142,
 0.8571428571428571,
 1.0,
 1.1428571428571428,
 1.2857142857142856,
 1.4285714285714284,
 1.5714285714285714,
 1.7142857142857142,
 1.857142857142857,
 2.0]

In [21]: type(b)
Out[21]: list

In [22]: type(b[0])
Out[22]: float

If, in fact, you really have python list of numpy.float64 objects, then @Alexander's answer is great, or you could convert the list to an array and then use the tolist() method. E.g.

In [46]: c
Out[46]: 
[0.0,
 0.33333333333333331,
 0.66666666666666663,
 1.0,
 1.3333333333333333,
 1.6666666666666665,
 2.0]

In [47]: type(c)
Out[47]: list

In [48]: type(c[0])
Out[48]: numpy.float64

@Alexander's suggestion, a list comprehension:

In [49]: [float(v) for v in c]
Out[49]: 
[0.0,
 0.3333333333333333,
 0.6666666666666666,
 1.0,
 1.3333333333333333,
 1.6666666666666665,
 2.0]

Or, convert to an array and then use the tolist() method.

In [50]: np.array(c).tolist()
Out[50]: 
[0.0,
 0.3333333333333333,
 0.6666666666666666,
 1.0,
 1.3333333333333333,
 1.6666666666666665,
 2.0]

If you are concerned with the speed, here's a comparison. The input, x, is a python list of numpy.float64 objects:

In [8]: type(x)
Out[8]: list

In [9]: len(x)
Out[9]: 1000

In [10]: type(x[0])
Out[10]: numpy.float64

Timing for the list comprehension:

In [11]: %timeit list1 = [float(v) for v in x]
10000 loops, best of 3: 109 µs per loop

Timing for conversion to numpy array and then tolist():

In [12]: %timeit list2 = np.array(x).tolist()
10000 loops, best of 3: 70.5 µs per loop

So it is faster to convert the list to an array and then call tolist().

Your code actually works fine for me (Python version 3.6). Try checking your Python version:

import sys
print(sys.version)    # Python 3.6.2 

If converting all columns to floats is the problem, try and use:

df = df.apply(np.float64)
df.dtypes

You should end up with:

A    float64
B    float64
C    float64
D    float64

You can convert most of the columns by just calling convert_objects:

In [36]:

df = df.convert_objects(convert_numeric=True)
df.dtypes
Out[36]:
Date         object
WD            int64
Manpower    float64
2nd          object
CTR          object
2ndU        float64
T1            int64
T2          int64
T3           int64
T4        float64
dtype: object

For column '2nd' and 'CTR' we can call the vectorised str methods to replace the thousands separator and remove the '%' sign and then astype to convert:

In [39]:

df['2nd'] = df['2nd'].str.replace(',','').astype(int)
df['CTR'] = df['CTR'].str.replace('%','').astype(np.float64)
df.dtypes
Out[39]:
Date         object
WD            int64
Manpower    float64
2nd           int32
CTR         float64
2ndU        float64
T1            int64
T2            int64
T3            int64
T4           object
dtype: object
In [40]:

df.head()
Out[40]:
        Date  WD  Manpower   2nd   CTR  2ndU   T1  　　T2   T3     T4
0   2013/4/6   6       NaN  2645  5.27  0.29  407   533  454    368
1   2013/4/7   7       NaN  2118  5.89  0.31  257   659  583    369
2  2013/4/13   6       NaN  2470  5.38  0.29  354   531  473  　　383
3  2013/4/14   7       NaN  2033  6.77  0.37  396   748  681    458
4  2013/4/20   6       NaN  2690  5.38  0.29  361   528  541    381

Or you can do the string handling operations above without the call to astype and then call convert_objects to convert everything in one go.

UPDATE

Since version 0.17.0 convert_objects is deprecated and there isn't a top-level function to do this so you need to do:

df.apply(lambda col:pd.to_numeric(col, errors='coerce'))

See the docs and this related question: pandas: to_numeric for multiple columns

The problem is that you do not do any type conversion of the numpy array. You calculate a float32 variable and put it as an entry into a float64 numpy array. numpy then converts it properly back to float64

Try someting like this:

a = np.zeros(4,dtype="float64") 
print a.dtype
print type(a[0])
a = np.float32(a)
print a.dtype
print type(a[0])

The output (tested with python 2.7)

float64
<type 'numpy.float64'>
float32
<type 'numpy.float32'>

a is in your case the array tree.tree_.threshold

Use val.item() to convert most NumPy values to a native Python type:

import numpy as np

# for example, numpy.float32 -> python float
val = np.float32(0)
pyval = val.item()
print(type(pyval))         # <class 'float'>

# and similar...
type(np.float64(0).item()) # <class 'float'>
type(np.uint32(0).item())  # <class 'int'>
type(np.int16(0).item())   # <class 'int'>
type(np.cfloat(0).item())  # <class 'complex'>
type(np.datetime64(0, 'D').item())  # <class 'datetime.date'>
type(np.datetime64('2001-01-01 00:00:00').item())  # <class 'datetime.datetime'>
type(np.timedelta64(0, 'D').item()) # <class 'datetime.timedelta'>
...

(A related method np.asscalar(val) was deprecated with 1.16, and removed with 1.23).

For the curious, to build a table of conversions of NumPy array scalars for your system:

for name in dir(np):
    obj = getattr(np, name)
    if hasattr(obj, 'dtype'):
        try:
            if 'time' in name:
                npn = obj(0, 'D')
            else:
                npn = obj(0)
            nat = npn.item()
            print('{0} ({1!r}) -> {2}'.format(name, npn.dtype.char, type(nat)))
        except:
            pass

There are a few NumPy types that have no native Python equivalent on some systems, including: clongdouble, clongfloat, complex192, complex256, float128, longcomplex, longdouble and longfloat. These need to be converted to their nearest NumPy equivalent before using .item().