Conditional average in Python

Question

I am having a problem manipulating my excel file in python. I have a large excel file with data arranged by date/time. I would like to be able to average the data for a specific time of day, over all the different days; ie. to create an average profile of the gas_concentrations over 1 day.

Here is a sample of my excel file:

Decimal Day of year     Decimal of day      Gas concentration   
133.6285                    0.6285           46.51230   
133.6493                    0.6493           47.32553   
133.6701                    0.6701           49.88705   
133.691                     0.691            51.88382   
133.7118                    0.7118           49.524 
133.7326                    0.7326           50.37112   

Basically I need a function, like the AVERAGEIF function in excel, that will say something like "Average the gas_concentrations when decimal_of_day =x" However I really have no idea how to do this. Currently I have got this far

import xlrd
import numpy as np

book= xlrd.open_workbook('TEST.xlsx')
level_1=book.sheet_by_index(0)

time_1=level_1.col_values(0, start_rowx=1, end_rowx=1088)
dectime_1=level_1.col_values(8, start_rowx=1, end_rowx=1088)
ozone_1=level_1.col_values(2, start_rowx=1, end_rowx=1088)
ozone_1 = [float(i) if i != 'NA' else 'NaN' for i in ozone_1]

Edit

I updated my script to include the following

ozone=np.array(ozone_1, float)
time=np.array(dectime_1)
a=np.column_stack((ozone, time))
b=np.where((a[:,0]<0.0035))
print b

EDIT Currently I solved the problem by putting both the variables into an array, then making a smaller array with just the variables I need to average - a bit inefficient but it works!

ozone=np.array(ozone_1, float)
time=np.array(dectime_1)
a=np.column_stack((ozone, time))
b=a[a[:,1]<0.0036]
c=np.nanmean(b[:,0])

Answer 1

You can use numpy masked array .

import numpy as np

data_1 = np.ma.arange(10)
data_1 = np.ma.masked_where(<your if statement>, data_1)

data_1_mean = np.mean(data1)

Hope that helps