Check none list python
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Python Python Null Example: What is None in PythonBy Krunal Last updated Aug 24, 2021 0 Python Null object is the singleton None. Theres nonull value in Python; instead, theres None. The equivalent of the null keyword in Python is None. Many would argue that the word null is somewhat esoteric. Its not exactly the friendliest word to programming novices. Also, None refers precisely to the intended functionality it is nothing and has no behavior. In Python, None is the object and first-class citizen! Show
History of NullMany programming languages use Null to represent a pointer that doesnt point to anything. Its a kind of placeholder when a variable is empty or to mark default parameters that you havent supplied yet. The Null is often defined to be 0 in those languages, but Null in Python is different. In most object-oriented languages, the naming of objects tends to use camel-case syntax. E.g. ThisIsMyObject. As youll see soon, Pythons None type is an object and behaves like one. As stated already, the most accurate way to test that something has been given None as the value is to use the is identity operator, which tests that two variables refer to the same object. In other programming languages, for example, this is how you may create a null variable in PHP and Java. Lets see how other Java and PHP define a Null variable. Null In JavamyVariable = null;Null In PHP$myVariable = NULL;If you need to evaluate a variable in the if condition, you may check this as follows in Java: if(myVariable == null) { System.out.println(Some output); }How to use the None in Python. I will use it in the if statement and a few compound data types. Understanding None in PythonIf you define a function that has a return statement or returns no value, then it returns None. def python_none(): pass print(python_none())OutputNoneWhen you call the python_none() function, it returns None because there is no return statement. Instead, there is only a pass statement. None has no output value, but printing shows None to the Python console that we are forcing explicitly to print the output. None indicates missing or default parameters. For example, None appears twice in the docs for list.sort. print(help(list.sort))Outputsort(self, /, *, key=None, reverse=False) Sort the list in ascending order and return None. The sort is in-place (i.e. the list itself is modified) and stable (i.e. the order of two equal elements is maintained). If a key function is given, apply it once to each list item and sort them, ascending or descending, according to their function values. The reverse flag can be set to sort in descending order.In the output, you can see that the parameter key has a None value. That means the key parameter is required, but we have not provided it; that is why the default parameter is None. Proof that None is ObjectNone is the singleton, and the NoneType class only gives you the same single instance of None. Theres only one None in our Python program. NoneObj = type(None)() print(NoneObj) # Check if NoneObj is instance of NoneType class print(NoneObj is None)It gives the following output. None TrueEven though you try to create the new instance, you still get the existing None. Interesting Facts about None
Declaring None in PythonWe can assign None object to any variable at the time of declaration. Python variables come to life by assignment operator, which helps us assign the particular value to the variable. In some programming languages, they dont have to have an initial value assigned to them. For example, the initial value for some types of variables might be Null. Lets assign the None value to a variable in Python and print the variable in the console. data = None print(None)OutputNoneIf we dont define a variable and direct print its value, then we will get NameError. print(data)OutputTraceback (most recent call last): File "app.py", line 2, inAll variables in Python come into life by the assignment operator. A variable can only start life as Null if you assign None to it. If you dont define a variable and directly try to print in the console, you will get NameError. Using None as part of a comparisonTheres no null in Python. Instead, theres None. As stated already, the most accurate way to test that something has been given None as a value is to use the is identity operator, which tests that two variables refer to the same object. In Python, to represent an absence of the value, you can use a Nonevalue (types.NoneType.None) for objects and(orlen() == 0) for strings. Regarding the difference between == and is, testing for object identity using == should be sufficient. However, since the operation is is defined as an object identity operation, it is probably more correct to use it, rather than ==. Not sure if there is even a speed difference. An essential rule of thumb keeps in mind when you are checking for None values.
The difference between == and Python is Operator.Difference between == and isoperatorinPython is that== operatorcompares the values of both the operands and checks for value equality. Whereas isoperatorchecks whether both the operands refer to the same object or not. See the following code. # app.py eleven = None print(eleven is None)See the following output. pyt python3 app.py True pytNow, see the following code. def python_none(): pass data = python_none() print(data == None) print(data is None)OutputTrue TrueIn this code, it returns None, and we check in both ways that data is indeed None. But, The equality operators can be fooled when youre comparing user-defined objects thatoverride them. class isAndDoubleEqual: def __eq__(self, other): return True res = isAndDoubleEqual() print(res == None) print(res is None)OutputTrue FalseThe equality operator == returns the incorrect answer. The identity is operator returns the correct answer because it cant be fooled because you cant override it. Checking type of NoneIn other programming languages, Null is just a synonym for 0, but Null in Python is an object. Check the type of None using the type() method. print(type(None))OutputThis shows that None is the object, and its data type is NoneType. None by itself is built into the language as the Null in Python. However, you can check it out by printing the dir(__builtins__). print(dir(__builtins__))It will print the following inbuilt items. ['ArithmeticError',..., None,....., zipUsing None as a Value in PythonIn Python, we can pass None as a function parameter and add it to the list. First, we will define a class, and we dont define its body. Then, we create a function called list_function(), and it takes two parameters.
Output[1, 2, 3, 4, 5]We have passed two parameters with its default value of the list_function().
In the code, we have passed the initial_list as data because we have a data list that has five elements. So, at the time of calling the function, our initial_list = data. That means, inside the function, we have checked if the initial_list is None or not, but it is not None as we have a list of items in data so that that condition will be False. The second condition is that new_element should not be DoAppendNone, but it is. So that condition will be False also. So, no new element will be added to the list. Finally, the provided data list will be returned as an output. Now, what if None is a valid input object? For instance, what if list_function() could either insert an item to the list or not, and None was a valid element to append? In this case, you can define the class specifically for use as default while being distinct from None. So, lets write a program that inserts a None value as an element to the list. class DoAppendNone: pass def list_function(new_item=DoAppendNone, initial_list=None): if initial_list is None: initial_list = [] if new_item is not DoAppendNone: initial_list.append(new_item) return initial_list data = [1, 2, 3, 4, 5] print(list_function(None, data))Output[1, 2, 3, 4, 5, None]Here, the class DoAppendNone serves as a signal not to append, so you dont want None for that. That frees us to insert None whenever you want. Checking None using ifelse condition.We can write the following code if we want to check if the value is None. # app.py value = None if value is None: print('The value is null') else: print('The value is not null')In the above example, we have defined the variable called value and assigns the None value. Then, we have used the ifelse condition to check the None value, and if it does, return the print statement with The value is null. We can use the following code if we want to check if the name exists. try: val except NameError: pass # val does not exist at allThere can be only one None in Python.The None is the inbuilt constant as soon as you start Python, whether in module, class, or function. The NonType, by contrast, is not; youd need to get a reference to it first by querying None forits class. >>> NoneType NameError: name 'NoneType' is not defined >>> type(None) NoneTypeNone can not be overwritten. Before Python 2.4, it was possible to reassign None, but not anymore. Not even as the class attribute or in the confines of a function. Checking if the value is NoneWhy do this value is Nonerather than value==NoneThe first is equivalent to: id(value)==id(None)Whereas the expression value==None is applied like this, value.__eq(None)__if the value is None, then youll get what you expected. >>> nothing = function_that_does_nothing() >>> nothing.__eq__(None) TrueThis is because none has a distinctive status in the Python language. Its a preferred baseline value because many algorithms treat it as an exceptional value. In such scenarios, it can be used as the flag to signal that the condition requires some special handling, such as the setting of the default value. None in TracebacksWhen NoneType appears in your traceback, it means that something you didnt expect to be None was None, and you tried to use it in a way that you cant use None. Almost always, its because youre trying to call a method on it. data = [1, 2, 3, 4, 5] data.append(6) print(data) data = None data.append(11) print(data)Output[1, 2, 3, 4, 5, 6] Traceback (most recent call last): File "app.py", line 7, inWhen we have defined a list with some initial values, then it can call the list.append() function to add an element to the list, but when we define a list to None, that means, now that list is not traditional Python list, that list is now None, and None object does not have an append() method to work with that is why it throws an AttributeError. When you see a traceback like the above in your code, look for the attribute that raised the error first. Here, it is an append() method. From there, youll see the object you tried to call it on. Finally, figure out how that object got to be None and take the necessary steps to fix your code. ConclusionNone in Python is an immutable keyword and powerful tool like True or False. None can be tested using in or not in operators. In tracebacks, None can be helpful to find the exact problem, and we can fix it by solving the Null issue. In any Python program, there can be only one None, and even if you create an instance of the NoneType class, there can only be one None. Finally, Python Null or None Example is over. Recommended PostsPython Not Equal Operator Python Absolute Value Python sqrt() Python stddev() Python variance() Python Null object is the singleton None. Theres nonull value in Python; instead, theres None. The equivalent of the null keyword in Python is None. Many would argue that the word null is somewhat esoteric. Its not exactly the friendliest word to programming novices. Also, None refers precisely to the intended functionality it is nothing and has no behavior. In Python, None is the object and first-class citizen! History of NullMany programming languages use Null to represent a pointer that doesnt point to anything. Its a kind of placeholder when a variable is empty or to mark default parameters that you havent supplied yet. The Null is often defined to be 0 in those languages, but Null in Python is different. In most object-oriented languages, the naming of objects tends to use camel-case syntax. E.g. ThisIsMyObject. As youll see soon, Pythons None type is an object and behaves like one. As stated already, the most accurate way to test that something has been given None as the value is to use the is identity operator, which tests that two variables refer to the same object. In other programming languages, for example, this is how you may create a null variable in PHP and Java. Lets see how other Java and PHP define a Null variable. Null In JavamyVariable = null;Null In PHP$myVariable = NULL;If you need to evaluate a variable in the if condition, you may check this as follows in Java: if(myVariable == null) { System.out.println(Some output); }How to use the None in Python. I will use it in the if statement and a few compound data types. Understanding None in PythonIf you define a function that has a return statement or returns no value, then it returns None. def python_none(): pass print(python_none())OutputNoneWhen you call the python_none() function, it returns None because there is no return statement. Instead, there is only a pass statement. None has no output value, but printing shows None to the Python console that we are forcing explicitly to print the output. None indicates missing or default parameters. For example, None appears twice in the docs for list.sort. print(help(list.sort))Outputsort(self, /, *, key=None, reverse=False) Sort the list in ascending order and return None. The sort is in-place (i.e. the list itself is modified) and stable (i.e. the order of two equal elements is maintained). If a key function is given, apply it once to each list item and sort them, ascending or descending, according to their function values. The reverse flag can be set to sort in descending order.In the output, you can see that the parameter key has a None value. That means the key parameter is required, but we have not provided it; that is why the default parameter is None. Proof that None is ObjectNone is the singleton, and the NoneType class only gives you the same single instance of None. Theres only one None in our Python program. NoneObj = type(None)() print(NoneObj) # Check if NoneObj is instance of NoneType class print(NoneObj is None)It gives the following output. None TrueEven though you try to create the new instance, you still get the existing None. Interesting Facts about None
Declaring None in PythonWe can assign None object to any variable at the time of declaration. Python variables come to life by assignment operator, which helps us assign the particular value to the variable. In some programming languages, they dont have to have an initial value assigned to them. For example, the initial value for some types of variables might be Null. Lets assign the None value to a variable in Python and print the variable in the console. data = None print(None)OutputNoneIf we dont define a variable and direct print its value, then we will get NameError. print(data)OutputTraceback (most recent call last): File "app.py", line 2, inAll variables in Python come into life by the assignment operator. A variable can only start life as Null if you assign None to it. If you dont define a variable and directly try to print in the console, you will get NameError. Using None as part of a comparisonTheres no null in Python. Instead, theres None. As stated already, the most accurate way to test that something has been given None as a value is to use the is identity operator, which tests that two variables refer to the same object. In Python, to represent an absence of the value, you can use a Nonevalue (types.NoneType.None) for objects and(orlen() == 0) for strings. Regarding the difference between == and is, testing for object identity using == should be sufficient. However, since the operation is is defined as an object identity operation, it is probably more correct to use it, rather than ==. Not sure if there is even a speed difference. An essential rule of thumb keeps in mind when you are checking for None values.
The difference between == and Python is Operator.Difference between == and isoperatorinPython is that== operatorcompares the values of both the operands and checks for value equality. Whereas isoperatorchecks whether both the operands refer to the same object or not. See the following code. # app.py eleven = None print(eleven is None)See the following output. pyt python3 app.py True pytNow, see the following code. def python_none(): pass data = python_none() print(data == None) print(data is None)OutputTrue TrueIn this code, it returns None, and we check in both ways that data is indeed None. But, The equality operators can be fooled when youre comparing user-defined objects thatoverride them. class isAndDoubleEqual: def __eq__(self, other): return True res = isAndDoubleEqual() print(res == None) print(res is None)OutputTrue FalseThe equality operator == returns the incorrect answer. The identity is operator returns the correct answer because it cant be fooled because you cant override it. Checking type of NoneIn other programming languages, Null is just a synonym for 0, but Null in Python is an object. Check the type of None using the type() method. OutputThis shows that None is the object, and its data type is NoneType. None by itself is built into the language as the Null in Python. However, you can check it out by printing the dir(__builtins__). print(dir(__builtins__))It will print the following inbuilt items. ['ArithmeticError',..., None,....., zipUsing None as a Value in PythonIn Python, we can pass None as a function parameter and add it to the list. First, we will define a class, and we dont define its body. Then, we create a function called list_function(), and it takes two parameters.
Output[1, 2, 3, 4, 5]We have passed two parameters with its default value of the list_function().
In the code, we have passed the initial_list as data because we have a data list that has five elements. So, at the time of calling the function, our initial_list = data. That means, inside the function, we have checked if the initial_list is None or not, but it is not None as we have a list of items in data so that that condition will be False. The second condition is that new_element should not be DoAppendNone, but it is. So that condition will be False also. So, no new element will be added to the list. Finally, the provided data list will be returned as an output. Now, what if None is a valid input object? For instance, what if list_function() could either insert an item to the list or not, and None was a valid element to append? In this case, you can define the class specifically for use as default while being distinct from None. So, lets write a program that inserts a None value as an element to the list. class DoAppendNone: pass def list_function(new_item=DoAppendNone, initial_list=None): if initial_list is None: initial_list = [] if new_item is not DoAppendNone: initial_list.append(new_item) return initial_list data = [1, 2, 3, 4, 5] print(list_function(None, data))Output[1, 2, 3, 4, 5, None]Here, the class DoAppendNone serves as a signal not to append, so you dont want None for that. That frees us to insert None whenever you want. Checking None using ifelse condition.We can write the following code if we want to check if the value is None. # app.py value = None if value is None: print('The value is null') else: print('The value is not null')In the above example, we have defined the variable called value and assigns the None value. Then, we have used the ifelse condition to check the None value, and if it does, return the print statement with The value is null. We can use the following code if we want to check if the name exists. try: val except NameError: pass # val does not exist at allThere can be only one None in Python.The None is the inbuilt constant as soon as you start Python, whether in module, class, or function. The NonType, by contrast, is not; youd need to get a reference to it first by querying None forits class. >>> NoneType NameError: name 'NoneType' is not defined >>> type(None) NoneTypeNone can not be overwritten. Before Python 2.4, it was possible to reassign None, but not anymore. Not even as the class attribute or in the confines of a function. Checking if the value is NoneWhy do this value is Nonerather than value==NoneThe first is equivalent to: id(value)==id(None)Whereas the expression value==None is applied like this, value.__eq(None)__if the value is None, then youll get what you expected. >>> nothing = function_that_does_nothing() >>> nothing.__eq__(None) TrueThis is because none has a distinctive status in the Python language. Its a preferred baseline value because many algorithms treat it as an exceptional value. In such scenarios, it can be used as the flag to signal that the condition requires some special handling, such as the setting of the default value. None in TracebacksWhen NoneType appears in your traceback, it means that something you didnt expect to be None was None, and you tried to use it in a way that you cant use None. Almost always, its because youre trying to call a method on it. data = [1, 2, 3, 4, 5] data.append(6) print(data) data = None data.append(11) print(data)Output[1, 2, 3, 4, 5, 6] Traceback (most recent call last): File "app.py", line 7, inWhen we have defined a list with some initial values, then it can call the list.append() function to add an element to the list, but when we define a list to None, that means, now that list is not traditional Python list, that list is now None, and None object does not have an append() method to work with that is why it throws an AttributeError. When you see a traceback like the above in your code, look for the attribute that raised the error first. Here, it is an append() method. From there, youll see the object you tried to call it on. Finally, figure out how that object got to be None and take the necessary steps to fix your code. ConclusionNone in Python is an immutable keyword and powerful tool like True or False. None can be tested using in or not in operators. In tracebacks, None can be helpful to find the exact problem, and we can fix it by solving the Null issue. In any Python program, there can be only one None, and even if you create an instance of the NoneType class, there can only be one None. Finally, Python Null or None Example is over. Recommended PostsPython Not Equal Operator Python Absolute Value Python sqrt() Python stddev() Python variance() |
