Reading with CSV files

CSV - Comma separated values

• 1st row in any csv file contains the header (what each column is)
• We use Python’s built-in CSV module that helps us read/write CSVs more easily

CSV modules

• reader
  • lets you iterate over rows of the CSV as lists
  • gives each row as lists
• DictReader
  • lets you iterate over rows of the CSV as OrderedDicts
  • each row is a an OrderedDict (dictionary with order)
• Both of them can accept csv files that aren’t separated by a comma (like a pipe |)
  • Then just csv_reader = reader(file, delimmiter ="|")

reader

Name,Country,Height (in cm)
Ryu,Japan,175
Ken,USA,175
Chun-Li,China,165
Guile,USA,182
E. Honda,Japan,185
Dhalsim,India,176
Blanka,Brazil,192
Zangief,Russia,214

from csv import reader
 
with open('fighters.csv') as file:
	csv_reader = reader(file) 
	next(csv_reader) #moves forward once to skip header
	for row in csv_reader:
		# each row is a list
		print(row)
 
# ['Name', 'Country', 'Height (in cm)']
# ['Ryu', 'Japan', '175']
# ...
# ['Zangief', 'Russia', '214']

• csv_reader is an iterator!
  • If you finish it and exhaust it, it won’t output anything
  • print(csv_reader) will give you <_csv.reader object at 0x7ff2f9149460>
• If you want to make the data into a list, then data = list(csv_reader)

DictReader

from csv import DictReader
 
with open('fighters/.csv') as file:
	csv_reader = DictReader(file) 
	next(csv_reader) #moves forward once to skip header
	for row in csv_reader:
		print(row)
 
# {'Name': 'Ken', 'Country': 'USA', 'Height (in cm)': '175'}
# {'Name': 'Chun-Li', 'Country': 'China', 'Height (in cm)': '165'}
# ...
# {'Name': 'Zangief', 'Country': 'Russia', 'Height (in cm)': '214'}

• OrderedDict
  • A special collection in Python
  • dictionary that is ordered
  • but we don’t have to use this because after python 3.7 dictionaries are all ordered!

Example

changing one dataset column - cm to inches

from csv import DictReader, DictWriter
 
def cm_to_in(cm):
	return float(cm) * 0.393701
	
with open("file1.csv") as f:
	csv_reader = DictReader(file)
	fighters = list(csv_reader) # to use for later
	# file1.csv has headers Name, Country, Height(cm)
 
with open('file2.csv') as f:
	headers = ["Name", "Country", "Height(in)"] # new header we will use
	csv_writer = DictWriter(file, fieldnames = headers)
	csv_writer.writeheader()
	for fighter in fighters: #looping through the list made earlier
		csv_writer.writerow({
			"Name": fighter["Name"],
			"Country": fighter["Country"],
			"Height(in)": cm_to_in(fighter["Height(cm)"])
		})

• DictReader automatically uses the 1st row of the CSV file as the header, so if you want to loop through your data just loop through from the start