So a few things that need to be changed/fixed:

• Syntax for your
  df = df.insert(0,thing.Date) needs to change to df.insert(0,thing.Date)
• Your def Date(): function doesn't actually do anything in the code. A print statement will output to the console so you can visually see what it's doing. But right now it's not returning anything usable when you do your insert. Fix that by adding the following:
  def Date(self): print ("2010-01") return "2010-01"
• Finally, the last issue with your code is the fact that you are trying to use the pandas package function .to_excel() on a list. The list df doesn't have a '.to_excel' function that can run on it. You have to use a Pandas dataframe object and to do that we have to convert your list into a dataframe like so:
  pd.DataFrame(df).to_excel("C:\\file2.xls")

So finally your code will look like this:

import os
import pandas as pd

df = []
for f in ['C:\\file.xls']:
    data = pd.read_excel(f, 'dspPrintOrExcel')
    data.index = [os.path.basename(f)]*len(data)

class Column(object):
        def Date(self):
            print ("2010-01")  # I kept your print in case you still wanted it
            return "2010-01"

thing = Column()

df.insert(0,thing.Date())

pd.DataFrame(df).to_excel("C:\\file2.xls")

What's the error?

So a few things that need to be changed/fixed:

• Syntax for your
  df = df.insert(0,thing.Date) needs to change to df.insert(0,thing.Date)
• Your def Date(): function doesn't actually do anything in the code. A print statement will output to the console so you can visually see what it's doing. But right now it's not returning anything usable when you do your insert. Fix that by adding the following:
  def Date(self): print ("2010-01") return "2010-01"
• Finally, the last issue with your code is the fact that you are trying to use the pandas package function .to_excel() on a list. The list df doesn't have a '.to_excel' function that can run on it. You have to use a Pandas dataframe object and to do that we have to convert your list into a dataframe like so:
  pd.DataFrame(df).to_excel("C:\\file2.xls")

So finally your code will look like this:

import os
import pandas as pd

df = []
for f in ['C:\\file.xls']:
    data = pd.read_excel(f, 'dspPrintOrExcel')
    data.index = [os.path.basename(f)]*len(data)

class Column(object):
        def Date(self):
            print ("2010-01")  # I kept your print in case you still wanted it
            return "2010-01"

thing = Column()

df.insert(0,thing.Date())

pd.DataFrame(df).to_excel("C:\\file2.xls")

You have a variable that is equal to None and you're attempting to access an attribute of it called 'something'.

Copyfoo = None
foo.something = 1

or

Copyfoo = None
print(foo.something)

Both will yield an AttributeError: 'NoneType'