Python:Assertion

Assertions in Python:

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What Are Assertions & What Are They Good For?

Python’s assert statement is a debugging aid that tests a condition. If the condition is true, it does nothing and your program just continues to execute. But if the assert condition evaluates to false, it raises an AssertionError exception with an optional error message.

The proper use of assertions is to inform developers about unrecoverable errors in a program. They’re not intended to signal expected error conditions, like “file not found”, where a user can take corrective action or just try again.

Another way to look at it is to say that assertions are internal self-checks for your program. They work by declaring some conditions as impossible in your code. If one of these conditions doesn’t hold that means there’s a bug in the program.

If your program is bug-free, these conditions will never occur. But if they do occur the program will crash with an assertion error telling you exactly which “impossible” condition was triggered. This makes it much easier to track down and fix bugs in your programs.

To summarize: Python’s assert statement is a debugging aid, not a mechanism for handling run-time errors. The goal of using assertions is to let developers find the likely root cause of a bug more quickly. An assertion error should never be raised unless there’s a bug in your program.

The assert Statement
When it encounters an assert statement, Python evaluates the accompanying expression, which is hopefully true. If the expression is false, Python raises an AssertionError exception.
The syntax for assert is –
assert Expression[, Arguments]
If the assertion fails, Python uses ArgumentExpression as the argument for the AssertionError. AssertionError exceptions can be caught and handled like any other exception using the try-except statement, but if not handled, they will terminate the program and produce a traceback.

Example1:
def get_age(age):
    print ("Your age is:" ,age)

get_age(20)
get_age(-1)

Output:
Your age is: 20
Your age is: -1


#After Adding Assert Staement
def get_age(age):
    assert age > 0, "Age cannot be Negative!"
    print ("Your age is:" ,age)
get_age(-1)

Output:
======
Traceback (most recent call last):
File "<ipython-input-27-6628f64fef17>", line 5, in <module>
get_age(-1)
File "<ipython-input-27-6628f64fef17>", line 2, in get_age
assert age > 0, "Age cannot be Negative!"
AssertionError: Age cannot be Negative!

Example2:
Here is a function that converts a temperature from degrees Kelvin to degrees Fahrenheit. Since zero degrees Kelvin is as cold as it gets, the function bails out if it sees a negative temperature.
#!/usr/bin/python
def KelvinToFahrenheit(Temperature):
   assert (Temperature >= 0),"Colder than absolute zero!"
   return ((Temperature-273)*1.8)+32

print KelvinToFahrenheit(273)
print int(KelvinToFahrenheit(505.78))
print KelvinToFahrenheit(-5)

When the above code is executed, it produces the following result −

32.0
451
Traceback (most recent call last):
   File "test.py", line 9, in <module>
      print KelvinToFahrenheit(-5)
   File "test.py", line 4, in KelvinToFahrenheit
      assert (Temperature >= 0),"Colder than absolute zero!"
AssertionError: Colder than absolute zero!

Go Through the Below Videos for a Quick Overview:
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