Python 3.8 Boolean String Comparison Bug

Question

Hi I'm an intermediate python user and I came across this strange bug. I tested it on both a Mac and Windows 10 computer and got the same results.

Here is my code:

def t1(time):
  return '21:00:00' < time

def t2(time):
  return '05:00:00' > time

def runScript(time):
  return '21:00:00' < time or '5:00:00' > time

def runScript2(time):
  return t1(time) or t2(time)

t = '15:00:00'

print(t1(t))
print(t2(t))
print(runScript(t))
print(runScript2(t))

Here are my results:

False
False
True
False

This is what I get running it straight from the interpreter

Python 3.8.0 (tags/v3.8.0:fa919fd, Oct 14 2019, 19:21:23) [MSC v.1916 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> def t1(time):
...   return '21:00:00' < time
...
>>> def t2(time):
...   return '05:00:00' > time
...
>>> def runScript(time):
...   return '21:00:00' < time or '5:00:00' > time
...
>>> def runScript2(time):
...   return t1(time) or t2(time)
...
>>> t = '15:00:00'
>>>
>>> print(t1(t))
False
>>> print(t2(t))
False
>>> print(runScript(t))
True
>>> print(runScript2(t))
False
>>> exit()

Thank you!

Answer 1

I guess you misinterpreted how string comparisons work in python and this clouded the actual solution for your problem.

You're trying to use date and time logic with strings, which is not recommendable. When using strings and operators you're doing a Lexicographical comparison:

Lexicographical ordering for strings uses the Unicode code point number to order individual characters.

So answering your first question, is not the boolean comparison in python that is wrong but how you wrote it:

def t2(time):
  # Here you wrote 05:00:00 as your string
  return '05:00:00' > time

def runScript(time):
  # Here you wrote 5:00:00 
  return '21:00:00' < time or '5:00:00' > time

In this case, '5:00:00' is different and greater them '05:00:00' as your interpreter is comparing these two strings not as a time data type but as a generic string.

# Using python 3.8
'05:00:00' > '5:00:00'
False
'5:00:00' > '05:00:00'
True

Python has a great datetime lib and you can find it's reference here: https://docs.python.org/3/library/datetime.html

from datetime import datetime

# datetime.strptime(date_string, format) convert's a string into a datetime object
def t1(time):
    time_string_t1 = '21:00:00'
    datetime_object_t1 = datetime.strptime(time_string_t1, '%H:%M:%S')
    return datetime_object_t1 < time

def t2(time):
    time_string_t2 = '05:00:00'
    datetime_object_t2 = datetime.strptime(time_string_t2, '%H:%M:%S')
    return datetime_object_t2 > time

def runScript(time):
    return  datetime.strptime('21:00:00', '%H:%M:%S') < time or datetime.strptime('05:00:00', '%H:%M:%S')> time

def runScript2(time):
    return t1(time) or t2(time)

t = datetime.strptime('15:00:00', '%H:%M:%S')

print(t1(t))
print(t2(t))
print(runScript(t))
print(runScript2(t))

Here I'm comparing two datetime objects, and the result was:

python test.py                                       
False
False
False
False

Please take a look in this solution the biggest problem in your code is that you're using the wrong data type for the problem you're trying to solve not the boolean logic.

Other useful references:

• String comparison technique used by Python
• https://docs.python.org/3/reference/lexical_analysis.html

Answer 2

In your runScript the second condition you are checking with '5:00:00' > time which supposed to be '05:00:00' > time because

('5:00:00' > '15:00:00') == True

Change your function to which is as function t2

>>> def runScript(time):
...   return '21:00:00' < time or '05:00:00' > time # '5:00:00'

Then it would be,

(False or False) == False

Answer 3

You are comparing strings. Strings compare with respect to lexical ordering, not to numerical ordering, and '5:00:00' (in runScript() is NOT the same as '05:00:00' (in t2())

Answer 4

change this line from

return '21:00:00' < time or '5:00:00' > time

to

return ('21:00:00' < time) or ('05:00:00' > time)

here the problem is you are not actually comparing a DateTime object so you can't expect 05:00:00 and 5:00:00 be same