How to Use the Pass Statement

How to use the pass statement

Suppose you are designing a new class with some methods that you don't want to implement, yet.

class MyClass(object):
    def meth_a(self):
        pass

    def meth_b(self):
        print "I'm meth_b"

If you were to leave out the pass, the code wouldn't run.

You would then get an:

IndentationError: expected an indented block

To summarize, the pass statement does nothing particular, but it can act as a placeholder, as demonstrated here.

How to use the pass statement

Suppose you are designing a new class with some methods that you don't want to implement, yet.

class MyClass(object):
    def meth_a(self):
        pass

    def meth_b(self):
        print "I'm meth_b"

If you were to leave out the pass, the code wouldn't run.

You would then get an:

IndentationError: expected an indented block

To summarize, the pass statement does nothing particular, but it can act as a placeholder, as demonstrated here.

Define Pass statement in Python?

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.

The pass statement in python acts as a placeholder for your future code. As the body of the loops and if condition cannot be empty in python, they cannot be left empty. However, it is possible that you have not yet decided what will happen if an if condition gets true. Then, you can use pass to avoid getting error and your code will not break.

# An if condition where the action is yet not known
if my_test_var == desired value:
    pass

What is the use case for pass in Python?

It is indeed useless in your example.

It is sometimes helpful if you want a block to be empty, something not otherwise allowed by Python. For instance, when defining your own exception subclass:

class MyException(Exception):
    pass

Or maybe you want to loop over some iterator for its side effects, but do nothing with the results:

for _ in iterator:
    pass

But most of the time, you won't need it.

Remember that if you can add something else that isn't a comment, you may not need pass. An empty function, for instance, can take a docstring and that will work as a block:

def nop():
    """This function does nothing, intentionally."""

Why I can't use pass in python if else statement when use condition in a single line

There is a difference between an if statement and a conditional expression.

When you write

if condition:
    this()
else:
    that()

you are writing a statement. It is an imperative construct — it does something. And the else bit is optional; if it is omitted, the statement will do nothing if the condition is not fulfilled.

The this if condition else that construct is an expression. It computes a value. It always has to compute a value, so the else bit has to be there. Using it for its side effects, for example by calling `list.append(), which doesn't return anything useful, is... not exactly wrong, but... well, it is wrong. Not technically, but philosophically.

So, when you want to compute a value, in one of two possible ways, and the whole thing fits in a line or so, use a conditional expression. If you want to do either of two things, one of which may be nothing, use an if statement.

Notice that you write

if temperature < 0:
    coat = thick
else:
    coat = thin

but

coat = thick if temperature < 0 else thin

In the first case, you have an if statement that selects which of two assignment statements to execute. In the second case, you have a single assignment statement, which uses a conditional expression to decide which value to assign.

Expressions can be used as statements (in which case their value is simply ignored), but statements have no value, and thus cannot be used as expressions.

What the purpose of the `pass` statement?

None already has meaning in Python. It is not a statement, but a value. It can be used to flag a variable that does not yet have a standard value, or as the "return value" of a function that does not return anything.

So using it as a statement would be more confusing than using the new keyword pass.

Pass statement in one-line Python if-else statement

pass is a statement, not a value, so it cannot be used in a conditional expression.

You want to use the filter clause of the list comprehension.

[col for col in cols if 'name' in col]

It is somewhat unfortunate that the keyword if is used in three distinct syntactic constructs (if statements, conditional expressions, and the iteration construct shared by list/dict/set comprehensions and generator expressions).

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