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I am learning python and have problem with logical operator 'or'. Can't find information and understand why code in various writing interpreted differently (what the difference in #First and #Second examples). So, #First example doesn't equal #Second, when operator 'and' equal in all variants of code

# First
a=35
b=35
if a or b>35:
    print('First')

'''with code like above python have printed 'First' even if statement false, like i understand it. 
But in other examples, statement are false too, and 'print' wasn't done, like it must be.'''

# Second
c=35
d=35
if c>35 or d>35:
    print('Second')

#Third    
e=35
f=35
if e>35 and f>35:
    print('Third')

#Fourth    
g=35
h=35
if g and h>35:
    print('Fourth')
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2
  • Please take look at python's operator precedence Commented May 1, 2020 at 13:34
  • You need to specify the condition for both of the conditions so instead of if a or b>35: it would be if a >35or b>35: Commented May 1, 2020 at 15:43

4 Answers 4

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5 
a or b > 35

is equivalent to

a or (b > 35)

and a is truthy because all integers except 0 are truthy. You can verify:

>>> a or b > 35
35
>>> (a or b) > 35  
False

See truth testing and operator precedence.

Also note that (a or b) > 35 will not become True if any of a or b is greater than 35. 

(a or b or c or ... or n) > 35

is true only if the first non-zero number in the disjunction is greater than 35

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Comments

2

"if a" will always evaluate to true for an int if a != 0.

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1

When you use a number X directly in a condition, Python implicitely converts it to a boolean value by applying the comparison X != 0

So, for your examples:

First

a=35
b=35
if a or b>35:        # same as if a!=0 or b>35:   True or False --> True
    print('First')

Second

c=35
d=35
if c>35 or d>35:     # False or False --> False
    print('Second')

Third

e=35
f=35
if e>35 and f>35:    # False and False --> False
    print('Third')

Fourth

g=35
h=35
if g and h>35:       # same as if g!=0 and h>35:  True and False --> False
    print('Fourth')

If you're looking to express a condition to test that either a or b are greater than 35, you cannot write it as a or b > 35.
While it may be clear to you in English, it is not how Python reads it.

Python requires that you be more explicit, so you will have to repeat the > 35 :

a > 35 or b > 35.

If you want to avoid repeating the 35, here's a trick you could use:

if max(a,b) > 35:  
   # either a or b is > 35  (i.e. the highest of the two is > 35)

if min(a,b) > 35:
   # both a and b are > 35  (i.e. the lowest of the two is > 35)
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0

Actually you have to split it to better understand it. Let's take the first case:

a=35
b=35
if a or b>35:
  print('First')

In this case, your if-condition is composed of two parts a and b>25, and the or operator means you need at least one of them to be True to print First.

So what does if a means? actually that means if a is equal to a value different than None, 0, "" and False. So in your case, a is equal 35 thus the evaluation of the first part is True. The second part is evaluated to False since b is not greater than 35. And this is why it prints First.

For the fourth example, you need both of them to be evaluated True to have the print statement executed.

I hope this helps you to better understand it.

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