| << Chapter < Page | Chapter >> Page > |
| Listing 3 . Blank lines and whitespace. |
|---|
a = 3
# spaces and commentb = 4
# tab and commentc = (a # continue on next line
+ b)# a blank line follows
print(c)# the output is 7 |
In every other programming environment that I have worked with in the past, indentation is used strictly for cosmetic or readability purposes.
However, indentation is not used strictly for cosmetic or readability purposes in Python. It is such an important topic in Python that I haveseparated it into a major section in this module.
Unlike most other programming environments, physical indentation is used in Python to determine the grouping of statements.
This can be both a blessing and a curse. The blessing is that it forces you to use proper indentation, which usually leads to more readable code.
The curse is that if you don't use proper indentation, your script probably won't behave properly.
There are few if any safety nets in Python (such as the matching curly brackets used in C, C++, and Java) to protect you from indentation errors.
To make matters worse, such errors often turn up as logical errors (meaning that the script simply doesn't work correctly) rather than interpreter errors (meaning that the interpreter will tell you about the error) .
Although I haven't introduced you to the if statement yet, you probably have a fairly good idea what it is used for. I am going to use it to illustrate theproper indentation of a group of statements with the script in Listing 4 .
An if statement means that if some expression evaluates to true , the program will do something specific. Otherwise, the script won't do that specificthing.
Don't be too concerned if the logic of the script in Listing 4 escapes you at this point in time. I will explain the use of the if statement in detail in a futuremodule. The important thing here is to understand the grouping of statements.
| Listing 4 . An example of correct indentation. |
|---|
A = 3
B = 4if B>A:
print(A) # begin groupprint(B)
print(A + B) # end groupA = 6 # not part of above group
print(A)#============================================================
#The output, which is not part of the script, is shown below.3
47
6 |
The script in Listing 4 compares the value of the variable A with the value of the variable B to determine if the value of B is greater than the value of A (if B>A:) .
If the value of B is greater than the value of A (which it is in this case) , the three indented print statements are executed. Otherwise, that group of threestatements is bypassed.
The three statements shown with indentation are either all executed, or they are all bypassed. Hence, they behave as a group.
A group of statements like this is sometimes referred to as a compound statement (a statement made up of two or more individual statements) .
When the three indented statements are executed as a group, the values 3, 4, and 7 are printed on consecutive output lines by the three print statements in the groupof statements, as shown in Figure 2 .
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