Python - opening reading writing files

From: LinkedIn course ‘Python Essential Training’ by Ryan Mitchell https://www.linkedin.com/learning/python-essential-training-14898805

reading files

f = open('some_file.txt','r')
print(f) #gets file type, need to read the file

f.readline()
f.readlines() #puts lines into list of strings
for line in f.readlines():
  print(line.strip()) # strips leading and trailing spaces

writing files

f = open('somefiles.txt','w') # creates a file
f.write('Line 1\n')
f.write('Line 2\n')
f.close() 
# python doesn't write until you close or run out of buffer and will overwrite existing text

appending files

f = open('somefile.txt', 'a')
f.write('Line 3\n')
f.write('Line 4\n')
f.close()

use ‘with’ so that the file closes when you finish the code block.

with open('somefiles.txt','a') as f: # creates a file
  f.write('Line 1\n')
  f.write('Line 2\n')

csv files

import csv

with open('somefile.csv','r') as f:
  # reader = csv.reader(f, delimiter = '\t') # for tab delimited ',' is assumed
  # reader = list(csv.reader(f, delimiter = '\t')) # read as list
  reader = csv.DictReader(f, delimiter = '\t') # read as dictionary
    for row in reader:
      print(row)

type(reader)

convert reader object to list object

with open('somefile.csv','r') as f:
  data - list(csv.DictReader(f. delimiter = '\t'))

Finding some prime location (zips that are prime in CA)

with open('geofile.csv','r') as f:
  data = list(csv.DictReader(f,delimiter='\t'))
  
primes = []
for number in range(2,99999):
  for factor in range(2, int(number**.05))
    if number % factor == 0:
      break
    else:
      primes.append(number)

data = [row for row in data if int(row['postal code']) in primes and row['state code'] == 'CA']

len(data)

writing the data to file

with open('somefile.csv','w') as f:
  writer = csv.writer(f)
  for row in data:
    writer.writerow([row['place name'], row['country']])

.json files

import json
from json import JSONDecodeError

jsonString = '{"a":"apple", "b":"bear", "c":"cat"}'
json.loads(jsonString)

## {'a': 'apple', 'b': 'bear', 'c': 'cat'}

# add trailing comma
jsonString = '{"a":"apple", "b":"bear", "c":"cat",}'

try:
  json.loads(jsonString)
except JSONDecodeError:
    print('could not parse JSON!')

## could not parse JSON!

Dumping JSON

pythonDict = {"a":"apple", "b":"bear", "c":"cat",}
json.dumps(pythonDict)

## '{"a": "apple", "b": "bear", "c": "cat"}'

and using custom decoder


from json import JSONEncoder

class Animal:
    def __init__(self,name):
      self.name = name
      
class AnimalEncoder(JSONEncoder):
  def default(self,o): #o is object that needs to be decoded
    if type(o) == Animal:
        return o.name
    return super().default(o)
    
pythonDict = {"a": Animal("aardvark"), "b" : Animal("bear"), "c" : Animal("cat"),}
json.dumps(pythonDict, cls=AnimalEncoder)

## '{"a": "aardvark", "b": "bear", "c": "cat"}'

Python

See also