python - CSV find max in column and append new data -

i asked question 2 hours ago regarding reading , writing of data website. i've spent last 2 hours since trying find way read maximum date value column 'a' of output, comparing value refreshed website data, , appending new data csv file without overriding old ones or creating duplicates.

the code 100% working this:

import requests symbol = "mtgoxusd" url = 'http://api.bitcoincharts.com/v1/trades.csv?symbol={}'.format(symbol) data = requests.get(url) open("trades_{}.csv".format(symbol), "r+") f:     f.write(data.text) 

i've tried various ways of finding maximum value of column 'a'. i've tried bunch of different ways of using "dict" , other methods of sorting/finding max, , using pandas , numpy libs. none of seem work. point me in direction of decent way find maximum of column .csv file? thanks!

i'll give 2 answers, 1 returns max value, , 1 returns row csv includes max value.

import csv import operator op import requests  symbol = "mtgoxusd" url = 'http://api.bitcoincharts.com/v1/trades.csv?symbol={}'.format(symbol) csv_file = "trades_{}.csv".format(symbol)  data = requests.get(url) open(csv_file, "w") f:     f.write(data.text)  open(csv_file) f:     next(f) # discard first row file -- see notes     max_value = max(row[0] row in csv.reader(f))  open(csv_file) f:     next(f) # discard first row file -- see notes     max_row = max(csv.reader(f), key=op.itemgetter(0)) 

notes:

• max() can directly consume iterator, , csv.reader() gives iterator, can pass in. i'm assuming might need throw away header line showed how that. if had multiple header lines discard, might want use islice() itertools module.

• in first one, use "generator expression" select single value each row, , find max. similar "list comprehension" doesn't build whole list, lets iterate on resulting values. max() consumes iterable , max value.

• max() can use key= argument specify "key function". use key function value , use value figure max... value returned max() unmodified original value (in case, row value csv). in case, key function manufactured operator.itemgetter()... pass in column want, , operator.itemgetter() builds function gets column.

the resulting function equivalent of:

def get_col_0(row):     return row[0] max_row = max(csv.reader(f), key=get_col_0) 

or, people use lambda this:

max_row = max(csv.reader(f), key=lambda row: row[0])

but think operator.itemgetter() convenient , nice read. , it's fast.

• i showed saving data in file, pulling file again. if want go through data without saving anywhere, need iterate on lines.

perhaps like:

text = data.text rows = [line.split(',') line in text.split("\n") if line] rows.pop(0)  # rid of first row data max_value = max(row[0] row in rows) max_row = max(rows, key=op.itemgetter(0)) 

• i don't know column want... column "a" might column 0 used 0 in above. replace column number like.