Sorting Strings of Dates in Python, Preferably Without Using Date Objects

One way is to use numpy.argsort combined with datetime library.

import numpy as np
from datetime import datetime

lst = ['March 1', 'March 9', 'April 14', 'March 12']

arr = np.array(lst)
res = arr[np.argsort([datetime.strptime(i+' 2018', '%B %d %Y') for i in lst])].tolist()

Result:

['March 1', 'March 9', 'March 12', 'April 14']

This is possible because, internally, dates are just numeric data. In this case, we attach an arbitrary year 2018 to create datetime objects.

You can also leverage the calendar module:

from calendar import month_name
months = list(month_name)

def parser (text):
    """Parses 'englishmonthname_whitespace_day-number' into string 'monthNR.dayNr'. 
    Will pad a zero to allow for string based sorting."""  
    try:
        month,day = text.split()
        monthAsIdx = months.index(month.strip())
        return '{:02d}.{:02d}'.format(monthAsIdx,int(day)) # return index in list.days
    except (ValueError, IndexError): # ValueError if not enough elements in string,
                                     # IndexError if not in list of month names
        return "99.99" # put last - all errors are put last w/o specific reordering

dates = ['TooFew', 'EnoughBut NotInList', 'March 1', 'March 9', 'April 14', 'March 12'] 

for n in dates:
    print(parser(n))


sortedDates = sorted(dates, key=lambda x: parser(x))

print(sortedDates)

Output:

# result of parser()
99.99
99.99
03.01
03.09
04.14
03.12

# sorted by key/lambda
['March 1', 'March 9', 'March 12', 'April 14', 'TooFew', 'EnoughBut NotInList'] 

You can use pandas module. Install it with pip.

You can do something like this:

import pandas as pd

dates = ['March 1', 'March 9', 'April 14', 'March 12']

df = pd.DataFrame(dates)
df = pd.to_datetime(df[0], format="%B %d")

df=df.sort_values() 

print (df)

This datetime format can be very useful like for example if you want the day or the month of an element of the list just do:

df.month
df.day

Possibilities:

• Use a dictionary and use key/value
• Use string matching (regex)
• Many more ...

Or google and use any of these:

• Converting string into datetime

• Convert string into Date type on Python

• how to convert a string date into datetime format in python?

To provide a possible solution:

Input_list = [{'month':'March', 'day':30}, {'month':'March', 'day':10}]

newlist = sorted(Input_list, key=lambda k: k['month'])