The if Statements

 The general form of an if statement looks like this:

if test1:                 # if test
    statements1           # Associated block
elif test2:               # Optional elifs
    statements2
else:                     # Optional else
    statements3

The one new syntax component in Python is the colon character (:). All Python compound statements—statements that have other statements nested inside them—follow the same general pattern of a header line terminated in a colon, followed by a nested block of code usually indented underneath the header line, like this:

Header line:
    Nested statement block

The colon is required, and omitting it is probably the most common coding mistake among new Python programmers

Most Python-friendly editors make this mistake easy to spot. Including it eventually becomes an unconscious habit (so much so that you may start typing colons in your C-like language code, too, generating many entertaining error messages from that language’s compiler!).

Although Python requires the extra colon character, there are three things programmers in C-like languages must include that you don’t generally have to in Python.

The first of these is the set of parentheses around the tests at the top of the statement:

if (x < y)

The parentheses here are required in all C-style languages including Perl. In Python, though, they are not—we simply omit the parentheses. Because every expression can be enclosed in parentheses, including them will not hurt, and they are not treated as an error if present. The “Python way” is to simply omit the parentheses in these kinds of statements altogether saving some typing and possible errors.

The second and more significant syntax component you won’t find in Python code is the semicolon. You don’t need to terminate statements with semicolons:

x = 1;

In Python, the general rule is that the end of a line automatically terminates the statement that appears on that line. In other words, you can leave off the semicolons.

x = 1

There are some ways to work around this rule, as you’ll see in a moment (for instance, wrapping code in a bracketed structure allows it to span lines). But, in general, you write one statement per line for the vast majority of Python code, and no semicolon is required.

 Semicolons are allowed but are not necessary in Python.

The elif and else clauses are optional. Python does not have a “switch” statement. Use if, elif, and else for all conditional processing.

End of indentation is end of block

Here’s a typical if statement with all three kinds of clauses (not in optimal style):

if a > b: 
   mymax=a
elif a<b: 
   mymax=b
else: 
  print 'a and b are even'

The block terminates when the indentation returns to that of the clause header (or further left from there). When there is just a single simple statement, as here, it can follow the : on the same logical line as the header, but it can also be on a separate logical line, immediately after the header line and indented rightward from it. Most Python programmers prefer the separate-line style, with four-space indents for the guarded statements. Such a style is more general and more readable, and recommended by PEP 8. So, a generally preferred style is:

if x < 0:
    print('x is negative')
elif x % 2:
    print('x is positive and odd')
else:
    print('x is even and non-negative')

You can use any Python expression as the condition in an if or elif clause. Using an expression this way is known as using it in a Boolean context. In a Boolean context, any value is taken as either true or false. As mentioned earlier, any nonzero number or nonempty container (string, tuple, list, dictionary, set) evaluates as true; zero (of any numeric type), None, and empty containers evaluate as false. To test a value x in a Boolean context, use the following coding style:

if x:

This is the clearest and most Pythonic form. Less clear forms are:

if x is True:
if x == True:
if bool(x):

There is a crucial difference between saying that an expression returns True (meaning the expression returns the value 1 with the bool type) and saying that an expression evaluates as true (meaning the expression returns any result that is true in a Boolean context). When testing an expression, for example in an if clause, you only care about what it evaluates as, not what, precisely, it returns.

When the if clause’s condition evaluates as true, the statements after the if clause execute, then the entire if statement ends. Otherwise, Python evaluates each elif clause’s condition, in order. The statements after the first elif clause whose condition evaluates as true, if any, execute, and the entire if statement ends. Otherwise, when an else clause exists it is executed, and then the statements after the entire if construct execute.

Python 3.8 introduced C-style operator := ( assignment expression or Walrus)

Python 3.8 implements the walrus operator ( C-style assignment expression), which looks like this:

name := expression

Slightly artificial but simple example (most commonly walrus is used when you search sub-string in the string and want to preserve the starting position of this search)  

my_list = [1,2,3,4,5]
if len(my_list) > 3:
   print(f"The list is too long with {len(my_list)} elements")

In this case the walrus operator  eliminates the need to  call the len() function twice.

my_list = [1,2,3,4,5]
if (n := len(my_list)) > 3:
   print(f"The list is too long with {n} elements")

The walrus also can be used with for  and while loops

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Last modified: July 28, 2020