Index#
- IndexError: List allocation index out of range
- (CSV) IndexError: List index out of range
- sys.argv[1] IndexError: List index out of range
- IndexError: Pop Index out of range
Make sure you click on the correct subheading based on the error message.
IndexError: List allocation index out of range in Python#
The Python error "IndexError: List Assignment Index Out of Range" occurs when trying to assign a value to an index that does not exist in the list.
To fix the error, use theattach ()Method to add an item to the end of the list, e.g.my_list.append('b').
Here is an example of how the error occurs.
main.py
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my list= ['A', 'B', 'C']# ⛔️ IndexError: List allocation index out of rangemy list[3] = 'D'
The list has a length of3. Since indexes are zero-based in Python, the first index is in the list0, and the last one is2.
| A | B | C |
|---|---|---|
| 0 | 1 | 2 |
Attempting to assign a value to a positive index out of range0-2would cause itindex error.
Adding an element to the end of the list with append()#
If you need to add an item to the end of a list, use thelist.append()instead.
main.py
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my list= ['A', 'B', 'C']my list.attach('D')my list.attach('e')press(my list) # 👉️ ['a', 'b', 'c', 'd', 'e']
Olist.append()The method adds an element to the end of the list.
The method returnsnonesince it changes the original list.
Changing item value at last list index#
If you want to change the value of the last index in the list, use-1.
main.py
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my list= ['A', 'B', 'C']my list[-1] = 'z'press(my list) # 👉️ ['a', 'b', 'z']
If the index starts with less, we start counting down from the end of the list.
Python indices are zero-based, so the first item in a list has an index of0, and the last element has an index of-1orlen(a_list) - 1.
Declaring a list containing N elements and updating a specific index#
Alternatively, you can declare a list with N elementsnoneValues.
main.py
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my list= [none] * 5press(my list) # 👉️ [None, none, none, none, none]my list[4] = 'Hello'press(my list) # 👉️ [None, none, none, none, 'Hello']
The specified item in the list will appear N times in the new list that the operation returns.
must not benone, can be the value0, an empty string, or any other value that fits your use case.
Be sure to include the value you want to repeat in a list.
If the list contains a value at a specific index, you can change it.
wearing atry/exceptInstructions for dealing with the error#
If you need to handle the error when the specified list index does not exist, use atry/exceptOpinion.
main.py
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my list= ['A', 'B', 'C']to try:my list[3] = 'D'exceptindex error: # 👇️ this works press('The specified mapping index DOES NOT exist')
The list in the example has 3 elements, so its last element has an index of2.
We pack the task into atry/exceptblock, then theindex erroris treated byexceptBlock.
You can use one tooto happendeclaration inexceptblock if you need to ignore the error.
main.py
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my list= ['A', 'B', 'C']to try:my list[3] = 'D'exceptindex error: to happen
Ostatement happendoes nothing and is used when an instruction is syntactically necessary but the program does not require any action.
Get the length of a list#
If you need to determine the length of the list, use thelen()Function.
main.py
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my list= ['A', 'B', 'C']press(len(my list)) # 👉️ 3
Olen()The function returns the length (the number of elements) of an object.
The argument the function accepts can be a sequence (a string, tuple, list, range, or bytes) or a collection (a dictionary, set, or frozen set).
If you need to check if an index exists before assigning a value, use aSeOpinion.
main.py
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my list= ['A', 'B', 'C']idx= 3Se len(my list) >idx:my list[idx] = 'Z' press(my list)anders: # 👇️ this works press(f'index{idx}is out of reach')
If a list has a length of3, then your last index2(because indexes are zero-based).
This means you can check if the length of the list is greater than the index you want to map to.
Attempting to assign a value to an empty list at a specific index#
Note that if you try to allocate an empty list at a specific index, you will always get aindex error.
main.py
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my list= []press(my list) # 👉️ []press(len(my list)) # 👉️ 0# ⛔️ IndexError: List allocation index out of rangemy list[0] = 'A'
You should print out the list you want to access and its size to ensure the variable stores what you expect.
Use oexpand()-Method to add multiple items to the end of a list#
If you need to add multiple items to the end of a list, use theexpand()Method.
Olista.extendThe method takes an iterable (eg a list) and extends the list by appending all elements of the iterable.
main.py
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my list= ['A', 'B']my list.expand(['C', 'D', 'e'])press(my list) # 👉️ ['a', 'b', 'c', 'd', 'e']
Olista.extendback methodnonesince it changes the original list.
(CSV) IndexError: List index out of range in Python#
The Python CSV "IndexError: list index out of range" occurs when trying to access a list with an out-of-range index, for example an empty line in a CSV file.
To fix the error, check that the line is not empty before accessing it in an index or check that the index is present in the list.
Here is an example of how the error occurs.
Suppose we have the followingCSVFile.
Employee.csv
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firstname, lastnameAlice,SmithBob,SmithCarl,Smith
And we try to read like that.
main.py
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mattercsvcom Open('employee.csv',New line='',codification='utf-8') withcsv file:csv_reader=csv.Reader(csv file,delimiter=',') forLineEmcsv_reader: # ⛔️ IndexError: List index out of range press(Line[0])
The second line ofCSVThe file is empty, so theLineThe variable stores an empty list on the second iteration.
Make sure the list contains items before accessing it#
One way to fix the error is to check the list for items before accessing it in an index.
main.py
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mattercsvcom Open('employee.csv',New line='',codification='utf-8') withcsv file:csv_reader=csv.Reader(csv file,delimiter=',') forLineEmcsv_reader: SeLine: # 👈️ checks if the row contains elements press(Line[0])
OSeThe statement checks whether the list is true on each iteration.
All values that do not match true are considered false. The incorrect values in Python are:
- as incorrectly defined constants:
noneEINCORRECT. 0(zero) of any numeric type- empty sequences and collections:
""(string vazia),()(double blank),[](empty list),{}(empty dictionary),Phrase()(empty set),Reach(0)(empty area).
Note that empty lists are false and lists containing at least 1 element are true.
Make sure the index you are trying to access is in the list#
Alternatively, you can check to see if the specific index you want to access is listed.
main.py
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mattercsvcom Open('employee.csv',New line='',codification='utf-8') withcsv file:csv_reader=csv.Reader(csv file,delimiter=',')idx= 1 forLineEmcsv_reader: Se len(Line) >idx: press(Line[idx])
Olen()The function returns the length (the number of elements) of an object.
The argument the function accepts can be a sequence (a string, tuple, list, range, or bytes) or a collection (a dictionary, set, or frozen set).
If a list has a length of2, then your last index1(because indexes are zero-based).
This means you can check if the length of the list is greater than the index you are trying to access.
use atry/exceptInstructions for dealing with the error#
Alternatively, you can also use atry/exceptblock to handle the error.
main.py
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mattercsvcom Open('employee.csv',New line='',codification='utf-8') withcsv file:csv_reader=csv.Reader(csv file,delimiter=',') forLineEmcsv_reader: to try: press(Line[1]) exceptindex error: press('ran out of the block') continue
We are trying to access the list for the current iteration in the index1, and if aindex erroris raised, we can handle itexceptblock or continue to the next iteration.
sys.argv[1] IndexError: List index out of range in Python#
The sys.argv "IndexError: list index out of range in Python" occurs when running a Python script without providing values for the required command line values.
To fix the error, provide values for the required arguments, for examplepython main.py first second.
Here is an example of how the error occurs.
main.py
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mattersystempress(system.argv) # 👉️ ['main.py']press(system.argv[0]) # 👉️ 'main.py'# ⛔️ IndexError: List index out of rangepress(system.argv[1])
I ran the script withpython-main.py.
Osys.argvlist contains the command line arguments passed to the Python script.
Woarg[0]is the name of the script,argument[1]is the first command-line argument given, and so on.
Provide any required command line arguments#
To fix the error, make sure you provide all the necessary command information when running the script, for example,python main.py first second.
main.py
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mattersystempress(system.argv) # 👉️ ['main.py', 'first', 'second']press(system.argv[0]) # 👉️ 'main.py'press(system.argv[1]) # 👉️ 'first'press(system.argv[2]) # 👉️ 'Monday'
Note that the first item in the list is always the name of the script.
It depends on the operating system whether this is the full path name or not.
Check if thesys.argvThe list contains the index#
If you don't always need to supply all the command line arguments your script is trying to access, use an if statement to check that thesys.argvlist contains the index you want to access.
main.py
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mattersystempress(system.argv) # 👉️ ['main.py']idx= 1Se len(system.argv) >idx: press(system.argv[idx])anders: # 👇️ this works press(f'index{idx}out of reach')
I ran the script like thispython-main.pywithout specifying any command-line values, so the condition is not met and theandersBlock the lie.
Olen()The function returns the length (the number of elements) of an object.
The argument the function accepts can be a sequence (a string, tuple, list, range, or bytes) or a collection (a dictionary, set, or frozen set).
If a list has a length of1, then your last index0(because indexes are zero-based).
This means you can check if the length of the list is greater than the index you are trying to access.
wearing atry/exceptInstructions for dealing with the error#
Alternatively, you can also use atry/exceptblock to handle the error.
main.py
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mattersystempress(system.argv) # 👉️ ['main.py']to try: press(system.argv[1])exceptindex error: # 👇️ this works press('index out of range')
We try to access the list item in the index1the one createdindex errorException.
You can handle the error or use theto happenkeyword inexceptBlock.
IndexError: Pop Index out of range in Python#
Python "IndexError: pop index out of range" occurs when passing an index that does not exist in the listPop()Method.
To correct the error, pass an existing index to the method or call thePop()Method without arguments to remove the last item from the list.
Here is an example of how the error occurs.
main.py
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my list= ['A', 'B', 'C']# ⛔️ IndexError: Pop Index out of rangeResult=my list.pop(3)
The list has a length of3. Since indices are zero-based in Python, the first item in the list has an index of0, and the latter an index of2.
| A | B | C |
|---|---|---|
| 0 | 1 | 2 |
If we pass a positive index outside the range0-2for thePop()method we would get aindex error.
If you need to remove the last item from the list, call the method without passing it an index.
main.py
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my list= ['A', 'B', 'C']Result=my list.pop()press(Result) # 👉️ 'c'# 👇️ ['a', 'b']press(my list)
Olist.popThe method removes and returns the item at the specified position in the list.
If no index is specified, thePop()The method removes and returns the last item in the list.
You can also use negative subscripts to count backwards, for examplemy_list.pop(-1)removes the last element from the list andmy_list.pop(-2)removes the penultimate element.
Alternatively, you can check whether an element exists at the specified index before passing it toPop().
main.py
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my list= ['A', 'B', 'C']press(len(my list)) # 👉️ 3idx= 3Se len(my list) >idx:Result=my list.pop(idx) press(Result)anders: # 👇️ this works press(f'index{idx}is out of reach')
Olen()The function returns the length (the number of elements) of an object.
The argument the function accepts can be a sequence (a string, tuple, list, range, or bytes) or a collection (a dictionary, set, or frozen set).
If a list has a length of3, then your last index2(because indexes are zero-based).
This means you can check if the length of the list is greater than the index you are passing it toPop().
heed the callPop()on an empty list also results in an error.
An alternative approach to handling the error is to use atry/exceptBlock.
main.py
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my list= ['A', 'B', 'C']idx= 3to try:Result=my list.pop(idx)exceptindex error: # 👇️ this works press(f'index{idx}is out of reach')
if you call themPop()The method with the given index triggers aindex error,To dieexceptblock is executed where we can handle the error or use theto happenkeyword to ignore it.