17 Time, Schedule Tasks, Launch Programmes

Question 1

Which code …

1. calculates how much time passed?
2. shows you your current time
3. rounds float numbers

Answer 1

import time

1.
>>>start = time.time()
>>>.... *wait*
>>>end = time.time()
>>>start - end

2. > > > time.ctime()

3. round(spam, nr), e.g.:
>>>spam = time.time()
>>>round(spam, 2)
>>>spam
1597662469.85

Question 2

How can you do arithmetic with dates or display them in a more convenient way?

1. Get current time
2. Save a certain day to a variable and then acess its year, month and day`
3. Can you compare “the size” of different datetime objects?

Answer 2

“datetime” module

1. > > > import datetime
   datetime.datetime.now()
   datetime.datetime(2020, 8, 17, 15, 33, 47, 126377)
2. > > > dt = datetime.datetime(2019,10, 21,16,29,0)
   dt.year, dt.month, dt.day
   (2019, 10, 21)

3. 
Yes - there is a sequence of time, which even took place earlier etc.
Thus:
==
!=
>

Question 3

How can you convert unix epoch timestamp to datetime?

What is a benefit of converting datetime?

Answer 3

datetime.datetime.fromtimestamp(epoch)

e.g.:
“epoch” could be 1.000.000 to receive the time 1.000.000 seconds after 1970
or it could be time.time() for the current time stamp

Benefit:
Datetime object is converted to current time zone

Question 4

Summarize the 3 Python Timefunctions

Answer 4

1. A Unix epoch timestamp (used by the time module) is a float or integer value of the number of seconds since 12 AM on January 1, 1970, UTC.
2. A datetime object (of the datetime module) has integers stored in the attributes year, month, day, hour, minute, and second.
3. A timedelta object (of the datetime module) represents a time duration, rather than a specific moment.

Question 5

Name 2x datetime and 2x timedelta functions incl. outcomes

Answer 5

Datetime:
datetime.datetime.now() This function returns a datetime object of the current moment.

datetime.datetime.fromtimestamp(epoch) This function returns a datetime object of the moment represented by the epoch timestamp argument.

Timedelta:
datetime.timedelta(weeks, days, hours, minutes, seconds, milliseconds, microseconds)
This function returns a timedelta object representing a duration of time. The function’s keyword arguments are all optional and do not include month or year.

total_seconds() This method for timedelta objects returns the number of seconds the timedelta object represents.

Question 6

Multiple Threading:

1. What is the advantage?
2. How can you create multiple Threads?

Answer 6

Advantage: Use computing or internet broadband capabilities fully

import threading, time
print(‘Start of program.’)

➊ def takeANap():
time.sleep(5)
print(‘Wake up!’)

➋ threadObj = threading.Thread(target=takeANap)
➌ threadObj.start()

print(‘End of program.’)

Question 7

What gets printed out last?

import threading, time
print(‘Start of program.’)

➊ def takeANap():
time.sleep(5)
print(‘Wake up!’)

➋ threadObj = threading.Thread(target=takeANap)
➌ threadObj.start()

print(‘End of program.’)

Answer 7

“Wake up!” is printed last, as it takes 5seconds to be printed.
The program only stops running once all threads reached their end.

Question 8

1. Can a new thread take arguments?
2. If so, how would you run this print call in its own thread?:
   »> print(‘Cats’, ‘Dogs’, ‘Frogs’, sep=’ & ‘)

Answer 8

1. Yes
2. The print call has 3 args and 1 kwarg “sep=’ & ‘”
   In threading.Thread():
   Arguments: Pass as a list
   Karguments: specify as dict

> > > import threading
threadObj = threading.Thread(target=print, args=[‘Cats’, ‘Dogs’, ‘Frogs’],
kwargs={‘sep’: ‘ & ‘})
threadObj.start()

Question 9

What are concurrency issues and how can you avoid them?

Answer 9

Issues that appear when multiple threads access and modify the same variables simultaneously. -> Hard to reproduce, difficult to debug

THUS:
Make sure new Thread Objects only access local variables

Question 10

How can you launch other programs with Python?

What 2 functions are useful in this context?

Answer 10

Popen(‘file’) -> P stands for process

> > > import subprocess
calc = subprocess.Popen(‘C:\Windows\System32\calc.exe’)

Useful 2:

calc. wait() # returns “0” once program is closed
calc. poll() # return None when program runs, “0” when closed

Question 11

How to pass cmd line arguments to Popen() Function

Answer 11

Pass a list as the sole arg to Popen()
- First string in list will be the executable filename of the program
-Subsequent strings will be cmd line args to pass the program when it starts
»> subprocess.Popen([‘C:\Windows\notepad.exe’, ‘C:\Users\Al\hello.txt’])

Question 12

How do you open Files with DEFAULT applications?

Answer 12

Some programs are already set up with an association, e.g. double-click .txt-file will open txt editor.
To start such a program on windows:

> > > import subprocess
subprocess.Popen([‘start’, ‘hello.txt’], shell=True)

Don´t forget “shell=True”!