Every programming language has some special words called keywords. The basic requirement to treat keywords is to avoid repetition when naming them. This article introduces the keywords in Python. Keywords are not built-in functions or built-in object types. Although it's better not to rename them, you can also use names that are the same as built-in functions or built-in object types. Keywords are different. They are not allowed.

In Python 3.8, 35 keywords are provided as follows:

False    await 	     else 	 import      pass
None     break       except      in          raise
True 	 class 	     finally     is    	     return
and 	 continue    for 	 lambda      try
as 	 def 	     from    	 nonlocal    while
assert 	 del 	     global   	 not 	     with
async 	 elif 	     if 	 or 	     yield

If you want to view keywords in interactive mode, you can use help():

>>> help("keywords")

Here is a list of the Python keywords.  Enter any keyword to get more help.

False     await        else        import        pass
None      break        except      in            raise
True      class        finally     is            return
and       continue     for         lambda        try
as        def          from        nonlocal      while
assert    del          global      not           with
async     elif         if          or            yield

For a detailed description of each keyword, you can also use help() to view:

>>> help('pass')    # The following appears after you tap enter

The "pass" statement
********************

   pass_stmt ::= "pass"

"pass" is a null operation — when it is executed, nothing happens. It
is useful as a placeholder when a statement is required syntactically,
but no code needs to be executed, for example:

   def f(arg): pass    # a function that does nothing (yet)

   class C: pass       # a class with no methods (yet)

In addition to the above methods, there is a standard library module keyword which provides keyword query function.

>>> import keyword
>>> keyword.kwlist
['False', 'None', 'True', 'and', 'as', 'assert', 'async', ...
>>> len(keyword.kwlist)
35

So, how to use these keywords? In what scenarios? Some key words are described in the following example.

True, False, and None

True and False are two values of boolean type, note that they must be capitalized.

>>> x = True
>>> x is True
True

>>> y = False
>>> y is False
True

If we want to determine whether the Boolean value of an object is True or False, we can use the bool() function, for example:

>>> x = "this is a truthy value"
>>> x is True
False
>>> bool(x) is True
True

>>> y = ""  # This is fake
>>> y is False
False
>>> bool(y) is False
True

Note that if the parameter passed in to bool() is 0, any one of '', {}, [], the return value is False.

In conditional statements, it is supposed to judge whether the condition is True or not. However, it is usually not necessary to directly compare it with True or False, relying on Python parser to automatically judge the condition.

>>> x = "this is a truthy value"
>>> if x is True:  # Don't do that
...     print("x is True")
...
>>> if x:  # It should be
...     print("x is truthy")
...
x is truthy

The keyword "None" means no value in Python. In other languages, the same meaning may be null,nil,none,undef,undefined, etc. None is also the default return value when there is no return statement in the function.

>>> def func():
...     print("hello")
...
>>> x = func()
hello
>>> print(x)
None

and,or,not,in,is

These keywords, in fact, correspond to operators in mathematics, as shown in the following table.

Python code has a strong readability. You can know what operation is at a glance through keywords.

These key words are easy to understand. Here we just remind you that there is a famous short circuit operation in Python, such as and:

<expr1> and <expr2>

Don't interpret the above formula as returning True when both sides are True. For this, please refer to the book "practical course of Python University". The other is or, which also has short circuit operation.

break, continue and else

These are the keywords that are often used in loop statements.

The function of break is to terminate the current loop. It uses the basic format:

for <element> in <container>:
    if <expr>:
        break
//for instance:

>>> nums = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> sum = 0
>>> for num in nums:
...     sum += num
...     if sum > 10:
...         break
...
>>> sum
15

Continue is to skip some loops and let them continue.

for <element> in <container>:
    if <expr>:
        continue

else is in the conditional statement. It is mentioned here. It is in the loop statement. Its function is to continue executing code when the loop ends.

In the for loop, use the following format:

for <element> in <container>:
    <statements>
else:
    <statements>

In the while loop, use the following format:

while <expr>:
    <statements>
else:
    <statements>

For example, sometimes we use a flag variable in a loop statement:

>>> for n in range(2, 10):
...     prime = True
...     for x in range(2, n):
...         if n % x == 0:
...             prime = False
...             print(f"{n} is not prime")
...             break
...     if prime:
...         print(f"{n} is prime!")
...
2 is prime!
3 is prime!
4 is not prime
5 is prime!
6 is not prime
7 is prime!
8 is not prime
9 is not prime

In the above code, prime is a flag variable. If the loop ends normally, the value of prime is True. Otherwise, it is False. If you exit from the loop, line 8 determines the value of this variable, and if it is True, the corresponding content will be printed.

For the above code, if you rewrite it with else, it can be simpler and more readable.

>>> for n in range(2, 10):
...     for x in range(2, n):
...         if n % x == 0:
...             print(f"{n} is not prime")
...             break
...     else:
...         print(f"{n} is prime!")
...
2 is prime!
3 is prime!
4 is not prime
5 is prime!
6 is not prime
7 is prime!
8 is not prime
9 is not prime