Real Python: The pass Statement: How to Do Nothing in Python

In Python, the pass keyword is an entire statement in itself. This statement doesn’t do anything: it’s discarded during the byte-compile phase. But for a statement that does nothing, the Python pass statement is surprisingly useful.

Sometimes pass is useful in the final code that runs in production. More often, pass is useful as scaffolding while developing code. In specific cases, there are better alternatives to doing nothing.

In this tutorial, you’ll learn:

• What the Python pass statement is and why it’s useful
• How to use the Python pass statement in production code
• How to use the Python pass statement as an aid while developing code
• What the alternatives to pass are and when you should use them

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Python pass Statement: Syntax and Semantics

In Python syntax, new indented blocks follow a colon character (:). There are several places where a new indented block will appear. When you start to write Python code, the most common places are after the if keyword and after the for keyword:

>>>

>>> for x in [1, 2, 3]:
...     y = x + 1
...     print(x, y)
...
1 2
2 3
3 4

After the for statement is the body of the for loop, which consists of the two indented lines immediately following the colon.

In this case, there are two statements in the body that are repeated for each value:

1. y = x + 1
2. print(x, y)

The statements inside this type of block are technically called a suite in the Python grammar. A suite must include one or more statements. It can’t be empty.

To do nothing inside a suite, you can use Python’s special pass statement. This statement consists of only the single keyword pass. While you can use pass in many places in Python, it’s not always useful:

>>>

>>> if 1 + 1 == 2:
...     print("math is ok")
...     pass
...     print("but this is to be expected")
...
math is ok
but this is to be expected

In this if statement, removing the pass statement would keep the functionality the same and make your code shorter. You might be wondering why the Python syntax includes a statement that tells the interpreter to do nothing. Couldn’t you achieve the same result by not writing a statement at all?

In some cases, explicitly telling Python to do nothing serves an important purpose. For example, because the pass statement doesn’t do anything, you can use it to fulfill the requirement that a suite include at least one statement:

>>>

>>> if 1 + 1 == 3:
...
  File "<stdin>", line 2

    ^
IndentationError: expected an indented block

Even if you don’t want to add any code inside the if block, an if block with no statement creates an empty suite, which is invalid Python syntax.

To fix this, you can use pass:

>>>

>>> if 1 + 1 == 3:
...     pass
...

Now, thanks to pass, your if statement is valid Python syntax.

Temporary Uses of pass

There are many situations in which pass can be useful to you while you’re developing, even if it won’t appear in the final version of your code. Much like scaffolding, pass can be handy for holding up the main structure of your program before you fill in the details.

It might sound strange to write code that will be deleted later, but doing things this way can accelerate your initial development.

Future Code

Read the full article at https://realpython.com/python-pass/ »

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