I am in the process of learning Python and I have reached the section about the pass statement. The guide I'm using defines it as being a null statement that is commonly used as a placeholder.
I still don't fully understand what that means though. What would be a simple/basic situation where the pass statement would be used and why would it be needed?
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.
Python has the syntactical requirement that code blocks (after if, except, def, class etc.) cannot be empty. Empty code blocks are however useful in a variety of different contexts, such as in examples below, which are the most frequent use cases I have seen.
Therefore, if nothing is supposed to happen in a code block, a pass is needed for such a block to not produce an IndentationError. Alternatively, any statement (including just a term to be evaluated, like the Ellipsis literal ... or a string, most often a docstring) can be used, but the pass makes clear that indeed nothing is supposed to happen, and does not need to be actually evaluated and (at least temporarily) stored in memory.
Ignoring (all or) a certain type of Exception (example from xml):
try:
self.version = "Expat %d.%d.%d" % expat.version_info
except AttributeError:
pass # unknown
Note: Ignoring all types of raises, as in the following example from pandas, is generally considered bad practice, because it also catches exceptions that should probably be passed on to the caller, e.g. KeyboardInterrupt or SystemExit (or even HardwareIsOnFireError – How do you know you aren't running on a custom box with specific errors defined, which some calling application would want to know about?).
try:
os.unlink(filename_larry)
except:
pass
Instead using at least except Error: or in this case preferably except OSError: is considered much better practice. A quick analysis of all Python modules I have installed gave me that more than 10% of all except ...: pass statements catch all exceptions, so it's still a frequent pattern in Python programming.
Deriving an exception class that does not add new behaviour (e.g., in SciPy):
class CompileError(Exception):
pass
Similarly, classes intended as abstract base class often have an explicit empty __init__ or other methods that subclasses are supposed to derive (e.g., pebl):
class _BaseSubmittingController(_BaseController):
def submit(self, tasks): pass
def retrieve(self, deferred_results): pass
Testing that code runs properly for a few test values, without caring about the results (from mpmath):
for x, error in MDNewton(mp, f, (1,-2), verbose=0,
norm=lambda x: norm(x, inf)):
pass
In class or function definitions, often a docstring is already in place as the obligatory statement to be executed as the only thing in the block. In such cases, the block may contain pass in addition to the docstring in order to say “This is indeed intended to do nothing.”, for example in pebl:
class ParsingError(Exception):
"""Error encountered while parsing an ill-formed datafile."""
pass
In some cases, pass is used as a placeholder to say “This method/class/if-block/... has not been implemented yet, but this will be the place to do it”, although I personally prefer the Ellipsis literal ... in order to strictly differentiate between this and the intentional “no-op” in the previous example. (Note that the Ellipsis literal is a valid expression only in Python 3)
For example, if I write a model in broad strokes, I might write
def update_agent(agent):
...
where others might have
def update_agent(agent):
pass
before
def time_step(agents):
for agent in agents:
update_agent(agent)
as a reminder to fill in the update_agent function at a later point, but run some tests already to see if the rest of the code behaves as intended. (A third option for this case is raise NotImplementedError. This is useful in particular for two cases: Either “This abstract method should be implemented by every subclass, and there isn't a generic way to define it in this base class”, or “This function, with this name, is not yet implemented in this release, but this is what its signature will look like”)
Besides its use as a placeholder for unimplemented functions, pass can be useful in filling out an if-else statement ("Explicit is better than implicit.")
def some_silly_transform(n):
# Even numbers should be divided by 2
if n % 2 == 0:
n /= 2
flag = True
# Negative odd numbers should return their absolute value
elif n < 0:
n = -n
flag = True
# Otherwise, number should remain unchanged
else:
pass
Of course, in this case, one would probably use return instead of assignment, but in cases where mutation is desired, this works best.
The use of pass here is especially useful to warn future maintainers (including yourself!) not to put redundant steps outside of the conditional statements. In the example above, flag is set in the two specifically mentioned cases, but not in the else-case. Without using pass, a future programmer might move flag = True to outside the condition—thus setting flag in all cases.
Another case is with the boilerplate function often seen at the bottom of a file:
if __name__ == "__main__":
pass
In some files, it might be nice to leave that there with pass to allow for easier editing later, and to make explicit that nothing is expected to happen when the file is run on its own.
Finally, as mentioned in other answers, it can be useful to do nothing when an exception is caught:
try:
n[i] = 0
except IndexError:
pass
The best and most accurate way to think of pass is as a way to explicitly tell the interpreter to do nothing. In the same way the following code:
def foo(x,y):
return x+y
means "if I call the function foo(x, y), sum the two numbers the labels x and y represent and hand back the result",
def bar():
pass
means "If I call the function bar(), do absolutely nothing."
The other answers are quite correct, but it's also useful for a few things that don't involve place-holding.
For example, in a bit of code I worked on just recently, it was necessary to divide two variables, and it was possible for the divisor to be zero.
c = a / b
will, obviously, produce a ZeroDivisionError if b is zero. In this particular situation, leaving c as zero was the desired behavior in the case that b was zero, so I used the following code:
try:
c = a / b
except ZeroDivisionError:
pass
Another, less standard usage is as a handy place to put a breakpoint for your debugger. For example, I wanted a bit of code to break into the debugger on the 20th iteration of a for... in statement. So:
for t in range(25):
do_a_thing(t)
if t == 20:
pass
with the breakpoint on pass.
A common use case where it can be used 'as is' is to override a class just to create a type (which is otherwise the same as the superclass), e.g.
class Error(Exception):
pass
So you can raise and catch Error exceptions. What matters here is the type of exception, rather than the content.
pass in Python basically does nothing, but unlike a comment it is not ignored by interpreter. So you can take advantage of it in a lot of places by making it a place holder:
1: Can be used in class
class TestClass:
pass
2: Can be use in loop and conditional statements:
if (something == true): # used in conditional statement
pass
while (some condition is true): # user is not sure about the body of the loop
pass
3: Can be used in function:
def testFunction(args): # The programmer wants to implement the body of the function later
pass
pass is mostly used when the programmer does not want to give implementation at the moment, but still wants to create a certain class/function/conditional statement which can be used later on. Since the Python interpreter does not allow for a blank or unimplemented class, function, or conditional statement it gives an error:
IndentationError: expected an indented block
pass can be used in such scenarios.
You can say that pass means a NOP (no operation) operation. You will get a clear picture after this example:
C Program
#include<stdio.h>
void main()
{
int age = 12;
if( age < 18 )
{
printf("You are not adult, so you can't do that task ");
}
else if( age >= 18 && age < 60)
{
// I will add more code later inside it
}
else
{
printf("You are too old to do anything , sorry ");
}
}
Now how you will write that in Python:
age = 12
if age < 18:
print "You are not adult, so you can't do that task"
elif age >= 18 and age < 60:
else:
print "You are too old to do anything , sorry "
But your code will give an error because it required an indented block after elif. Here is the role of the pass keyword.
age = 12
if age < 18:
print "You are not adult, so you can't do that task"
elif age >= 18 and age < 60:
pass
else:
print "You are too old to do anything , sorry "
Now I think it's clear to you.
Honestly, I think the official Python docs describe it quite well and provide some examples:
The pass statement does nothing. It can be used when a statement is required syntactically but the program requires no action. For example:
>>> while True: ... pass # Busy-wait for keyboard interrupt (Ctrl+C) ...This is commonly used for creating minimal classes:
>>> class MyEmptyClass: ... pass ...Another place pass can be used is as a place-holder for a function or conditional body when you are working on new code, allowing you to keep thinking at a more abstract level. The pass is silently ignored:
>>> def initlog(*args): ... pass # Remember to implement this! ...
If you want to import a module, if it exists, and ignore importing it, if that module does not exists, you can use the below code:
try:
import a_module
except ImportError:
pass
# The rest of your code
If you avoid writing the pass statement and continue writing the rest of your code, a IndentationError would be raised, since the lines after opening the except block are not indented.
As the book said, I only ever use it as a temporary placeholder, i.e.,
# code that does something to to a variable, var
if var == 2000:
pass
else:
var += 1
And then later fill in the scenario where var == 2000.
In addition to the "main" use of "I don't want anything to go here," here's one I just found, assuming gen is an Iterator:
i = 0
obj = None
for i, obj in enumerate(gen):
pass
This gets the last object yielded from gen and the length of gen, all in one pass. (Sorta similar to a while loop that ends with a semicolon in C, come to think of it.)
Pass is usually helpful when you are writing code to design a game or something of that type , now as you proceed with the code , you would realize that , there are some functions you don't want to move ahead and want to leave it untouched and move further on with the code and then revisit the function later , but as you do so , the compiler would obviously thrown an error saying it isn't either recognized and all sorts of other things!! In this condition you use pass so that the compiler would simply neglect the function and move further on with the code !!
For example -
if __name__ = "__main__":
pass
print("Hello World!")
Here the compiler would neglect the if name part and would print Hello World as directed !!
Thanks !
First, if you want to write a block, like this:
if statement:
pass
for i in range(abc):
pass
def func():
pass
And pass can to be a placeholder.
Second, it can let you 'communicate' with IDE:
When you want to let your IDE decrement indentations like this:
If your program written here:
class abc(parent):
def __init__(self, params):
self.params=params
if d:
return
else:
return
# cursor in there
Now your indentations count is 2, but you want it is 1 in next line.
You can type a pass, and your program to be this:
class abc(parent):
def __init__(self, params):
self.params=params
if d:
return
else:
return
pass# cursor in there
And return. It will let you happy:
class abc(parent):
def __init__(self, params):
self.params=params
if d:
return
else:
return
pass
# cursor in there
Now the indentation count is 1.
pass is just the indication code of emptiness.
For example, pass is used to create a empty class or function as shown below:
class Test:
pass
def test():
pass
But, if a class or function really doesn't have anything even pass as shown below:
class Test:
# pass
def test():
# psss
There is an error as shown below:
SyntaxError: unexpected EOF while parsing
And as I said before, pass is just the indication code of emptiness so if there is some code after pass, the code works as shown below:
class Test:
pass
x = "Hello World"
def test():
pass
return "Hello World"
print(Test.x) # Hello World
print(test()) # Hello World
The pass statement in Python is used when a statement is required syntactically, but you do not want any command or code to execute.
The pass statement is a null operation; nothing happens when it executes. The pass is also useful in places where your code will eventually go, but has not been written yet (e.g., in stubs for example):
Example:
#!/usr/bin/python
for letter in 'Python':
if letter == 'h':
pass
print 'This is pass block'
print 'Current Letter :', letter
print "Good bye!"
This will produce the following result:
Current Letter : P
Current Letter : y
Current Letter : t
This is pass block
Current Letter : h
Current Letter : o
Current Letter : n
Good bye!
The preceding code does not execute any statement or code if the value of letter is 'h'. The pass statement is helpful when you have created a code block, but it is no longer required.
You can then remove the statements inside the block, but let the block remain with a pass statement so that it doesn't interfere with other parts of the code.
Pass refers to ignore...as simple as it is. If the given condition is true and the next statement is pass, it ignores that value or iteration and proceed to the next line.
Example:
for i in range (1, 100):
if i%2 == 0:
pass
else:
print(i)
Output: Prints all the odd numbers from 1-100
This is because the modulus of an even number is equal to zero, hence it ignores the number and proceeds to next number. Since odd numbers' modulus are not equal to zero, the else part of the loop is executed and it's printed.
pass is used to avoid indentation errors in Python.
If we take languages like C, C++, and Java, they have braces like:
if(i==0)
{}
else
{//some code}
But in Python we use indentation instead of braces, so to avoid such errors we use pass.
Remembered as you were playing a quiz and
if(dont_know_the_answer)
pass
Example program,
for letter in 'geeksforgeeks':
pass
print 'Last letter: ', letter
