Openpyxl: Manipulation of cell values - python-3.x

Question

I'm trying to pull cell values from an excel sheet, do math with them, and write the output to a new sheet. I keep getting an ErrorType. I've run the code successfully before, but just added this aspect of it, thus code has been distilled to below:
import openpyxl
#set up ws from file, and ws_out write to new file
def get_data():
first = 0
second = 0
for x in range (1, 1000):
if ws.cell(row=x, column=1).value == 'string':
for y in range (1, 10): #Only need next ten rows after 'string'
ws_out.cell(row=y, column=1).value = ws.cell(row=x+y, column=1).value
second = first #displaces first -> second
first = ws.cell(row=x+y, column=1).value/100 #new value for first
difference = first - second
ws_out.cell(row=x+y+1, column=1).value = difference #add to output
break
Throws a TypeError message:
first = ws.cell(row=x+y, column=1).value)/100
TypeError: unsupported operand type(s) for /: 'NoneType' and 'int'
I assume this is referring to the ws.cell value and 100, respectively, so I've also tried:
first = int(ws.cell(row=x, column=1))/100 #also tried with float
Which raises:
TypeError: int() argument must be a string or a number
I've confirmed that every cell in the column is made up of numbers only. Additionally, openpyxl's cell.data_type returns 'n' (presumably for number as far as I can tell by the documentation).
I've also tested more simple math, and have the same error.
All of my searching seems to point to openpyxl normally behaving like this. Am I doing something wrong, or is this simply a limitation of the module? If so, are there any programmatic workarounds?
As a bonus, advice on writing code more succinctly would be much appreciated. I'm just beginning, and feel there must be a cleaner way to write an ideas like this.
Python 3.3, openpyxl-1.6.2, Windows 7
Summary
cfi's answer helped me figure it out, although I used a slightly different workaround. On inspection of the originating file, there was one empty cell (which I had missed earlier). Since I will be re-using this code later on columns with more sporadic empty cells, I used:
if ws.cell(row=x+r, column=40).data_type == 'n':
second = first #displaces first -> second
first = ws.cell(row=x+y, column=1).value/100 #new value for first
difference = first - second
ws_out.cell(row=x+y+1, column=1).value = difference #add to output
Thus, if a specified cell was empty, it was ignored and skipped.

Answer 1

Are you 100% sure (=have verified) that all the cells you are accessing actually hold a value? (Edit: Do a print("dbg> cell value of {}, {} is {}".format(row, 1, ws.cell(row=row, column=1).value)) to verify content)
Instead of going through a fixed range(1,1000) I'd recomment to use openpyxl introspection methods to iterate over existing rows. E.g.:
wb=load_workbook(inputfile)
for ws in wb.worksheets:
for row in ws.rows:
for cell in row: value = cell.value
When getting the values do not forget to extract the .value attribute:
first = ws.cell(row=x+y, column=1).value/100 #new value for first
As a general note: x, and y are useful variable names for 2D coordinates. Don't use them both for rows. It will mislead others who have to read the code. Instead of x you could use start_row or row_offset or something similar. Instead of y you could just use row and you could let it start with the first index being the start_row+1.
Some example code (untested):
def get_data():
first = 0
second = 0
for start_row in range (1, ws.rows):
if ws.cell(row=start_row, column=1).value == 'string':
for row in range (start_row+1, start_row+10):
ws_out.cell(row=start_row, column=1).value = ws.cell(row=row, column=1)
second = first
first = ws.cell(row=row, column=1).value/100
difference = first - second
ws_out.cell(row=row+1, column=1).value = difference
break
Now with this code I still don't understand what you are trying to achieve. Is the break indented correctly? If yes, the first time you are matching string, the outer loop will be quit by the break. Then, what is the point of the variables first and second?
Edit: Also ensure that your reading from and writing into cell().value not just cell().