Extract n characters for the first match of a word in a file - python-3.x

Question

I am a beginner in Python. I have a file having single line of data. My requirement is to extract "n" characters after certain words for their first occurrence only. Also, those words are not sequential.
Data file: {"id":"1234566jnejnwfw","displayId":"1234566jne","author":{"name":"abcd#xyz.com","datetime":15636378484,"displayId":"23423426jne","datetime":4353453453}
I want to fetch value after first match of "displayId" and before "author", i.e.; 1234566jne. Similarly for "datetime".
I tried breaking the line based upon index as the word and putting it into another file for further cleaning up to get the exact value.
tmpFile = "tmpFile.txt"
tmpFileOpen = open(tmpFile, "w+")
with open("data file") as openfile:
for line in openfile:
tmpFileOpen.write(line[line.index(displayId) + len(displayId):])
However, I am sure this is not a good solution to work further.
Can anyone please help me on this?

Answer 1

This answer should work for any displayId with a similar format as in your question. I decided not to load the JSON file for this answer, because it wasn't needed to accomplish the task.
import re
tmpFile = "tmpFile.txt"
tmpFileOpen = open(tmpFile, "w+")
with open('data_file.txt', 'r') as input:
lines = input.read()
# Use regex to find the displayId element
# example: "displayId":"1234566jne
# \W matches none words, such as " and :
# \d matches digits
# {6,8} matches digits lengths between 6 and 8
# [a-z] matches lowercased ASCII characters
# {3} matches 3 lowercased ASCII characters
id_patterns = re.compile(r'\WdisplayId\W{3}\d{6,8}[a-z]{3}')
id_results = re.findall(id_patterns, lines)
# Use list comprehension to clean the results
clean_results = ([s.strip('"displayId":"') for s in id_results])
# loop through clean_results list
for id in clean_results:
# Write id to temp file on separate lines
tmpFileOpen.write('{} \n'.format(id))
# output in tmpFileOpen
# 1234566jne
# 23423426jne
This answer does load the JSON file, but this answer will fail if the JSON file format changes.
import json
tmpFile = 'tmpFile.txt'
tmpFileOpen = open(tmpFile, "w+")
# Load the JSON file
jdata = json.loads(open('data_file.txt').read())
# Find the first ID
first_id = (jdata['displayId'])
# Write the first ID to the temp file
tmpFileOpen.write('{} \n'.format(first_id))
# Find the second ID
second_id = (jdata['author']['displayId'])
# Write the second ID to the temp file
tmpFileOpen.write('{} \n'.format(second_id))
# output in tmpFileOpen
# 1234566jne
# 23423426jne

Answer 2

If I understand correctly your question, you can achieve this by doing the following:
import json
tmpFile = "tmpFile.txt"
tmpFileOpen = open(tmpFile, "w+")
with open("data.txt") as openfile:
for line in openfile:
// Loads the json to a dict in order to manipulate it easily
data = json.loads(str(line))
// Here I specify that I want to write to my tmp File only the first 3
// characters of the field `displayId`
tmpFileOpen.write(data['displayId'][:3])
This can be done because the data in your file is JSON, however if the format changes it won't work