deleting characters using backspace - python-3.x

Question

I'm creating a text box class with a method that updates its text. It has the user's keyboard input as parameter, keys. The text box class also has an attribute, text, which is pretty self explanatory, it's its text. So I need to add the users input to the text box's text. Except if the user types a backspace key (which is '\x08') I'll need to delete all the letters before it. But of course if there's 2 backspaces followed up consecutively, it's not like a backspace would delete a backspace, I'll need to delete twice instead. So now that you understand the situation, here's my approach to the problem:
def update(self, keys):
if len(keys) > 0:
# I have a shift variable because when you modify a variable at the same time as
# iterating through it, the index will off shift, so I need to reverse that
shift = 0
for i in range(len(keys)):
i -= shift
# If there's a backspace
if keys[i] == '\x08':
# If it's the first character
if i == 0:
# Remove the last character from the text box's text
self.text = self.text[:-1]
# Get rid of the backspace from the user input
keys = keys[1:]
# reverse the off shift, or else the for loop will skip the second element
shift += 1
else:
j = i
# Counts how many backspaces follow up consecutively
del_count = 1
while keys[j] != '\x08':
j -= 1
del_count += 1
# Adjusts the user's input
keys = keys[:j - del_count] + keys[i+1:]
shift += del_count
self.text += keys
print(self.text)
It almost works perfectly, except when the user's input is a character followed up by multiple backspaces, I get an index error. For example, if the user input (keys) was "a\x08\x08", I'd get an index error at if keys[i] == '\x08':. What is the cause for this? And can someone give me a solution? Thanks in advance.

Answer 1

You should not modify the iterable inside its loop.
I was approached this problem within 3 steps:
Merged text and keys first
Count and collect index of backspace chars and effected normal chars
Deleted both of them in the merged object in reversed order
Try this out:
def update(text, keys):
if keys:
# Step 1: Merge both `key` and `texts`
print(repr(text))
text += keys
print(repr(text))
# Step 2: Count and collect `backspace` posision and `effected chars` position in reversed order
backspace_posision = []
char_deleted_position = []
for i, char in enumerate(text[::-1]):
_r_i = - i - 1
if char == "\x08":
backspace_posision.append(_r_i)
print("backspace_posision:", backspace_posision)
elif len(char_deleted_position) < len(backspace_posision):
char_deleted_position.append(_r_i)
print(char)
print("char_deleted_position:", char_deleted_position)
# Step 3: Remove them in reversed order
texts = list(text)
text_leng = len(texts)
print(repr(texts))
for _r_i in sorted(backspace_posision + char_deleted_position, reverse=True):
i = text_leng + _r_i
print(i)
texts.pop(i)
print(repr(texts))
text = "".join(texts)
print(repr(text))
Example output will look like this
update("ABC", "D\x08\x08EFG\x08H\x08\x08")
# Original text
>>>'ABC'
# Merged
>>>'ABCD\x08\x08EFG\x08H\x08\x08'
# Collecting backspace and effected chars in reversed order
# Collecting backspace
>>>backspace_posision: [-1]
>>>backspace_posision: [-1, -2]
# Collecting effected chars
>>>H
>>>char_deleted_position: [-3]
# Collecting backspace
>>>backspace_posision: [-1, -2, -4]
# Collecting effected chars
>>>G
>>>char_deleted_position: [-3, -5]
>>>F
>>>char_deleted_position: [-3, -5, -6]
# Collecting backspace
>>>backspace_posision: [-1, -2, -4, -8]
>>>backspace_posision: [-1, -2, -4, -8, -9]
# Collecting effected chars
>>>D
>>>char_deleted_position: [-3, -5, -6, -10]
>>>C
>>>char_deleted_position: [-3, -5, -6, -10, -11]
# Iterate the merged text in reversed order and remove them
>>>['A', 'B', 'C', 'D', '\x08', '\x08', 'E', 'F', 'G', '\x08', 'H', '\x08', '\x08']
>>>12
>>>['A', 'B', 'C', 'D', '\x08', '\x08', 'E', 'F', 'G', '\x08', 'H', '\x08']
>>>11
>>>['A', 'B', 'C', 'D', '\x08', '\x08', 'E', 'F', 'G', '\x08', 'H']
>>>10
>>>['A', 'B', 'C', 'D', '\x08', '\x08', 'E', 'F', 'G', '\x08']
>>>9
>>>['A', 'B', 'C', 'D', '\x08', '\x08', 'E', 'F', 'G']
>>>8
>>>['A', 'B', 'C', 'D', '\x08', '\x08', 'E', 'F']
>>>7
>>>['A', 'B', 'C', 'D', '\x08', '\x08', 'E']
>>>5
>>>['A', 'B', 'C', 'D', '\x08', 'E']
>>>4
>>>['A', 'B', 'C', 'D', 'E']
>>>3
>>>['A', 'B', 'C', 'E']
>>>2
>>>['A', 'B', 'E']
>>>'ABE'