parsing a text file into lists with python

Here is an example that you could build off of, every time it reads a string rather than a number it then makes a new list and puts all the dates under the group in it

import os

#read file
lineList = 0
with open("test.txt") as f:
  lineList = f.readlines()

#make new list to hold variables
lists = []

#loop through and check for numbers and strings
y=-1
for x in range(len(lineList)):
    #check if it is a number or a string
    if(lineList[x][0] is not None and not lineList[x][0].isdigit()):
        #if it is a string make a new list and push back the name
        lists.append([lineList[x]])
        y+=1
    else:
        #if it is the number append it to the current list
        lists[y].append(lineList[x])

#print the lists
for x in lists:
    print(x)

Start by reading in your whole text file. Then you can count the amount of occurrences of "DateGroup", which seems to be the constant part in your date group separation. You can then parse your file by going through all the data that is in between any two "DateGroup" identifiers or between one "DateGroup" identifier and the end of the file. Try to understand the following piece of code and build your application on top of that:

file = open("dates.txt")
text = file.read()
file.close()

amountGroups = text.count("DateGroup")

list = []

index = 0
i = 0
for i in range(amountGroups):
    list.append([])

    index = text.find("DateGroup", index)
    index = text.find("\n", index) + 1
    indexEnd = text.find("DateGroup", index)
    if(indexEnd == -1):
        indexEnd = len(text)
    while(index < indexEnd):
        indexNewline = text.find("\n", index)
        list[i].append(text[index:indexNewline])
        index = indexNewline + 1

print(list)

This first section just to show how to treat a string with data as if it came from a file. That helps if you don't want to generate the actual file of the OP but want to visibly import the data in the editor.

import sys
from io import StringIO  # allows treating some lines in editor as if they were from a file)

dat=StringIO("""DateGroup1
20191129
20191127
20191126
DateGroup2
20191129
20191127
20191126
DateGroup3
2019-12-02
DateGroup4
2019-11-27
DateGroup5
2019-11-27""")

lines=[ l.strip() for l in dat.readlines()]    
print(lines) 

output:

   ['DateGroup1', '20191129', '20191127', '20191126', 'DateGroup2', '20191129', '20191127', '20191126', 'DateGroup3', '2019-12-02', 'DateGroup4', '2019-11-27', 'DateGroup5', '2019-11-27']

Now one possible way to generate your desired list of lists, while ensuring that both possible date formats are covered:

from datetime import datetime
b=[]
for i,line in enumerate(lines):
    try:             # try first dateformat
        do = datetime.strptime(line, '%Y%m%d')
        a.append(datetime.strftime(do,'%Y-%m-%d'))
    except:
        try:         # try second dateformat
            do=datetime.strptime(line,'%Y-%m-%d')
            a.append(datetime.strftime(do,'%Y-%m-%d'))
        except:       # if neither date, append old list to list of lists  & make a new list
            if a!=None:
                b.append(a)
            a=[]
    if i==len(lines)-1:
        b.append(a)

b

output:

 [['2019-11-27'],
 ['2019-11-29', '2019-11-27', '2019-11-26'],
 ['2019-11-29', '2019-11-27', '2019-11-26'],
 ['2019-12-02'],
 ['2019-11-27'],
 ['2019-11-27']]

TTP can help to parse this text as well, here is sample template with code how to run it:

from ttp import ttp

data_to_parse = """
DateGroup1
20191129
20191127
20191126
DateGroup2
20191129
20191127
20191126
DateGroup3
2019-12-02
DateGroup4
2019-11-27
DateGroup5
2019-11-27
"""

ttp_template = """
<group name="date_groups.date_group{{ id }}">
DateGroup{{ id }}
{{ dates | to_list | joinmatches() }}
</group>
"""

parser = ttp(data=data_to_parse, template=ttp_template)
parser.parse()
print(parser.result(format="json")[0])

above code would produce this output:

[
    {
        "date_groups": {
            "date_group1": {
                "dates": [
                    "20191129",
                    "20191127",
                    "20191126"
                ]
            },
            "date_group2": {
                "dates": [
                    "20191129",
                    "20191127",
                    "20191126"
                ]
            },
            "date_group3": {
                "dates": [
                    "2019-12-02"
                ]
            },
            "date_group4": {
                "dates": [
                    "2019-11-27"
                ]
            },
            "date_group5": {
                "dates": [
                    "2019-11-27"
                ]
            }
        }
    }
]

This is my attempt to parse that text data. I deliberately chose parsec.py, a haskell parsec-like parser combinators library, because it works more clearly then regular expressions, so it is easier to debug and test. And second cause is much more flexibility of getting output data format.

import re
from parsec import *

spaces = regex(r'\s*', re.MULTILINE)

@generate
def getHeader():
  s1 = yield string ("DateGroup") 
  s2 = ''.join( (yield many1(digit())))
  return (s1 + s2)

@generate
def getDataLine():
  s1 = yield digit()
  s2 = ''.join((yield many1 (none_of ("\r\n"))))
  yield spaces
  return (s1 + s2)

@generate
def getChunk():
  yield spaces
  header = yield getHeader
  yield spaces
  dataList = yield many1 (getDataLine)
  return (header,dataList)

@generate
def getData():
  yield spaces
  parsedData = yield many1(getChunk)
  yield eof()
  return parsedData

inputText = """DateGroup1
20191129
20191127
20191126
DateGroup2
20191129
20191127
20191126
DateGroup3
2019-12-02
DateGroup4
2019-11-27
DateGroup5
2019-11-27"""


result = getData.parse(inputText)
for p in result:
  print(p)

Output:

('DateGroup1', ['20191129', '20191127', '20191126'])
('DateGroup2', ['20191129', '20191127', '20191126'])
('DateGroup3', ['2019-12-02'])
('DateGroup4', ['2019-11-27'])
('DateGroup5', ['2019-11-27'])