Python loop does not work accurately and reliably with time.sleep - python-3.x

Question

What would be the best way to run a program once a day.
time.sleep is inaccurate on long runs.
I try to get def send_stock_data (self): run once a day, for example. time.sleep drifts too much for this purpose.
The program reads the excel file and sends the data by e-mail if the column 'Amount' is 0.
class progloop():
def prog_loop (self):
progress.pack_forget()
while run_cycle == True:
get_stock_and_send_mail()
sleep(48000)
continue
def __init__(self):#adds Threading to loop
t = threading.Thread(target=self.prog_loop)
t.start()
The above class currently handles the loop.
Below is the full script.
import threading
import tkinter as tk
import os
import smtplib
import datetime
import time
import pandas as pds
import traceback
import warnings
warnings.simplefilter(action='ignore', category=FutureWarning)
from tkinter import BOTTOM,HORIZONTAL, Label, Menu, PhotoImage, messagebox, filedialog, ttk
from datetime import datetime
from time import sleep
root = tk.Tk()
sec = 0
action = None
run_cycle = True
stop_cycle = True
today = datetime.now()
clock_time = today.strftime('%H:%M:%S')
date = today.strftime('%d/%m/%Y')
weekday = datetime.today().strftime('%A')
def on_run():#start send_stock_data loop and timer
global run_cycle, action
global action
string_var.set('Application state: Running')
run_cycle = True
if not action:
timer()
def on_stop():#pause send_stock_data loop and timer
global run_cycle, action
string_var.set('Application state: Paused')
run_cycle = False
messagebox.showinfo('Attention','Program is paused, press Run to resume execution')
if action:
root.after_cancel(action)
action = None
def Info():#Info btn data
messagebox.showinfo('Application info','''-The application reads the stock balance from the excel file and sends out e-mail information if the part (s)
of the warehouse balance has run out of stock within the alarm limit.\n
-The default path to the excel file is specified in the Windows environment variables as 'FilePath'\n
-In case of problems contact the author
\n\nDefault alarm limit = 0 parts in stock
\n\nDefault Google smtp server:'smtp.gmail.com'\nDefault port: 587 ''')
def write_err_log():#writes an error log with traceback
traceback_err = traceback.format_exc()
date_time = [clock_time,date]
write_log = open("ErrorLog.txt", "a")
write_log.write(str(date_time)) and write_log.write(str(traceback_err))
write_log.close()
#Gui
string_var = tk.StringVar()
string_var.set('Application state: Waiting for commands')
root.iconbitmap(r'C:\\Users\\Lauri\\Visual_studio\\Bitmap_2.ico')
root.title('Warehouse assistant')
root.geometry('460x360')
root.resizable(False,False)
background_image = PhotoImage(file = r'C:\\Users\\Lauri\\Visual_studio\\BcImage.png')
background_label = tk.Label(image=background_image)
background_label.place(x=0, y=0, relwidth=1, relheight=1)
Label_header = ttk.Label(root, font=('underline', 12),text='Parts balance monitoring application').pack()
Label_start = ttk.Label(root,font=('underline',8), text='Press Run to start the application\nPress Pause to stop the application').pack()
Label_appState = ttk.Label(root, font=('underline', 10),textvariable=string_var).pack()
btnInfo = ttk.Button(root, text = 'Info', width= 15, command = Info).pack()
Start_btn = ttk.Button(root, text = 'Run', width= 15, command=lambda:[bar(),progloop(),root.after(1000,on_run())]).pack()
Stop_btn = ttk.Button(root, text = 'Pause', width= 15, command = on_stop).pack()
exit_btn = ttk.Button(root, text = 'Exit', width= 25, command =lambda:[root.after(1000,),root.destroy()]).pack(side=BOTTOM)
menu = Menu(root)
root.config(menu=menu)
contact_num = os.environ.get('Contact_number')
contact_email = os.environ.get('Contact_email')
helpmenu = Menu(menu)
menu.add_cascade(label='Contact', menu=helpmenu)
helpmenu.add_command(label=contact_email)
helpmenu.add_command(label=contact_num)
helpmenu.add_command(label= 'Exit', command=root.destroy)
progress = ttk.Progressbar(root, orient = HORIZONTAL,length = 158, mode = 'determinate')
def bar():#progressbar
progress['value'] = 5
root.update_idletasks()
root.after(50)
progress['value'] = 43
root.update_idletasks()
root.after(300)
progress['value'] = 68
root.update_idletasks()
root.after(50)
progress['value'] = 100
root.update_idletasks()
root.after(1)
progress.pack(side=BOTTOM)
class get_stock_and_send_mail: #send inventory balance information by email
def send_stock_data(self):
count = 0
pds.set_option('display.max_colwidth', -1)
pds.set_option('colheader_justify','left')
#reads excel file and searches for desired inventory balance value
while True:
file = os.environ.get('FilePath')#default path to file specified in the Windows environment variables as 'FilePath'
try:
dataFrame = pds.read_excel(file)
break
except PermissionError as error:
write_err_log()
messagebox.showerror(title='error',message= 'Permission Error Permission error has occured during read file?\n\n\n'+str(error)+"'.",
detail=traceback.format_exc())
except FileNotFoundError as error:
messagebox.showwarning(title='Select Excel File',
message='DEFAULT FILE NOT FOUND.\nChoose Excel file from directory.\nCONTINUE --> PRESS OK')
file = filedialog.askopenfilename()#manual path to file if default path doesn't exist
try:
dataFrame = pds.read_excel(file)
break
except FileNotFoundError:
write_err_log()
continue
df_bool = []
for Amount in dataFrame.Amount:
if Amount <= 0:
df_bool.append(True)
else:
df_bool.append(False)
if True in df_bool:
while run_cycle == True:
today = datetime.now()
clock_time = today.strftime('%H:%M:%S')
date = today.strftime('%d/%m/%Y')
weekday = datetime.today().strftime('%A')
Email_adress = os.environ.get('email_user')
Email_password = os.environ.get('email.password')
empty_stock = dataFrame.loc[dataFrame.Amount <= 0]
try:
with smtplib.SMTP('smtp.gmail.com',587) as server:
server.ehlo()
server.starttls()
server.ehlo_or_helo_if_needed()
server.set_debuglevel(1)
server.login(Email_adress,Email_password)
subject = 'no-reply'
body = f"""Row no.:{empty_stock}\nPart (s) is out of stock\n
Date: {weekday} {date}\n
Time: {clock_time}
\n\n\nThis was an automatically generated message.\nDo not reply to this message!
"""
msg = f'subject:{subject}\n\n{body}'
server.sendmail(Email_adress,Email_adress, msg)
server.quit
except smtplib.SMTPException as error:
write_err_log()
count +=1
if count <= 5:
continue
elif count >= 6:
messagebox.showerror(title='error',message= 'ERROR'+str(error)+"'.",
detail=traceback.format_exc())
break
def __init__(self):#adds Threading to loop
t = threading.Thread(target=self.send_stock_data)
t.start()
class progloop():
def prog_loop (self):
progress.pack_forget()
while run_cycle == True:
get_stock_and_send_mail()
sleep(48000)
continue
def __init__(self):#adds Threading to loop
t = threading.Thread(target=self.prog_loop)
t.start()
def timer(): #Runtime timer
global sec
if run_cycle == False:
return
sec += 1
time['text'] = f'Runtime: {sec} sec'
time.after(1000, timer)
time = Label(root, fg='black')
time.pack(side=BOTTOM)
root.mainloop()

Answer 1

If you really want to actually use a sleep, and time shifting/drifting over multiple days is the problem, what you can do, although not perfectly accurate, is to readjust the sleep time based on current time. Something like :
def seconds_since_midnight():
now = datetime.now()
return (now - now.replace(hour=0, minute=0, second=0, microsecond=0)).total_seconds()
def one_day_to(day_time_secs):
return 86400+day_time_secs-seconds_since_midnight()
And using for instance
sleep(one_day_to(3600*18))
to schedule at about 18h00 the next day, without drifting from day to day