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TypeError: 'list' object cannot be interpreted as an integer, when using a regex. How can I fix this?

I know there are similar threads to this question (having looked at them already) but I cannot, as a noob, work out how to translate those answers across to adjust my script to make it work (4+ days of trying).
So.. I have a python script to randomly select a subset of items from a file and components of those items. I want to create two new txt files as output. One with the subset of items and one with just a list of components (Ingredients) for those items.
To do this I have done write-lines to the first txt (MenuOutput.txt)file and then want to use regex (re.sub) to strip out the first part of the string from each line in the second file (ShoppingOutput.txt).
Now the issue: the TypeError: 'list' object cannot be interpreted as an integer. I understand (I think) the problem is the re.sub outputs a list object. But I don't know another way to strip the first part of each line from a text file. Is there a way of tweaking the re.sub to make it work, or do I need another function I am unaware of?
Menu_choices = random.sample(sample_list, k=6)
MenuOutput = open('MenuOutput.txt', 'w')
for element in Menu_choices:
MenuOutput.write(element)
MenuOutput.close()
MyFile = open('ShoppingOutput.txt', 'w')
ShoppingOutput = re.sub(r'.*?', 'I', Menu_choices)
for element in ShoppingOutput:
MyFile.write(element)
MyFile.close

Just like you loop over the list of strings to write them, you have to loop over them to perform other string manipulations on them.
with open('ShoppingOutput.txt', 'w') as my_file:
for element in MenuChoices:
my_file.write(re.sub(r'.*?', 'I', element))
Notice also the upgrade to a with statement, and using snake_case for regular variables.
Your regex seems both inexact and inefficient, though. Probably better to just my_file.write('I' + element)) and get rid of the no-op re.sub, or perhaps replace with a simple substring operation if the intent was to remove a prefix but you hadn't worked out the correct regex for that yet.
my_file.write('I' + element[element.index(' ')+1:])
would write everything after the first space.

How do I write and structure code in most efficient way possible?

Few weeks ago I needed a crawler for data collection and sorting so I started learning python.
Same day I wrote a simple crawler but the code looked ugly as hell. Mainly because I don't know how to do certain things and I don't know how to properly google them.
Example:
Instead of deleting [, ] and ' in one line I did
extra_nr = extra_nr.replace("'", '')
extra_nr = extra_nr.replace("[", '')
extra_nr = extra_nr.replace("]", '')
extra_nr = extra_nr.replace(",", '')
Because I couldn't do stuff to list object and when I did str(list object) It looked like ['this', 'and this'].
Now I'm creating discord bot that will upload data that I feed to it to google spreadsheet. The code is long and ugly. And it takes like 2-3 secs to start the bot (idk if this is normal, I think the more I write the more time it takes to start it which makes me think that code is garbage). Sometimes it works, sometimes it doesn't.
My question is how do I know that I wrote something good? And if I just keep adding stuff like in the example, how will it affect my program? If I have a really long code do I split it and call the parts of it only when they are needed or how does it work?

tl;dr to get good at Python and write good code, write a lot of Python and read other people's code. Learn multiple approaches to different problem types and get a feel for which to use and when. It's something that comes over time with a lot of practice. As far as resources, I highly recommend the book "Automate the Boring Stuff with Python".
As for your code sample, you could use translate for this:
def strip(my_string):
bad_chars = [*"[],'"]
return my_string.translate({ord(c): None for c in bad_chars})
translate does a character by character translation of the string given a translation table, so you create a small translation table with the characters you don't want set to None.
The list of characters you don't want is created by unpacking (splatting) a string of the characters.
>>> [*"abc"] == ["a", "b", "c"]
True
Another option would be using comprehensions:
def strip(my_string):
bad_chars = [*"[],'"]
return "".join(c for c in my_string if c not in bad_chars)
Here we use the comprehension format [x for x in y] to build a new list of xs from y, just specifying to drop the character if it appears in bad_chars. We then join the remaining list of characters into a string that doesn't have the specified characters in it.

You will definitely improve quickly from reading (or listening) up on Python best practices from resources like Real Python and Talk Python To Me.
Meanwhile, I'd recommend starting using some code analysers like pylint and bandit as part of your regular workflow.
In any case, welcome to the world of Python and enjoy! :-)

You can use maketrans() to define characters to remove (3rd parameter):
def clean(S): return S.translate(str.maketrans("","","[],'"))
clean("A['23']") # 'A23'

How to change a 1 line output to multiple lines

I have this line in a .txt file:
I just got home.\nDid you make lunch?\nOh, I see...
It's just one line, it's formatted like this for a game, so it is exactly like this, with the \n's.
What I need is that when I print this, after I store it into a variable or list, to print it on multiple lines, respecting the \n. Like this:
I just got home.\n
Did you make lunch?\n
Oh, I see...
I can't think of a way to modify the output to make a new line after the program displays each \n. How would I do this?

Using of str.replace() function would be enough:
s="I just got home.\nDid you make lunch?\nOh, I see..."
print(s.replace("\n", "\\n\n"))
The output:
I just got home.\n
Did you make lunch?\n
Oh, I see...

Python : splitting and splitting

I need some help;
I'm trying to program a sort of command prompt with python
I need to split a text file into lines then split them into strings
example :
splitting
command1 var1 var2;
command2 (blah, bleh);
command3 blah (b bleh);
command4 var1(blah b(bleh * var2));
into :
line1=['command1','var1','var2']
line2=['command2']
line2_sub1=['blah','bleh']
line3=['blah']
line3_sub1=['b','bleh']
line4=['command4']
line4_sub1=['blah','b']
line4_sub2=['bleh','var2']
line4_sub2_operand=['*']
Would that be possible at all?
If so could some one explain how or give me a piece of code that would do it?
Thanks a lot,

It's been pointed out, that there appears to be no reasoning to your language. All I can do is point you to pyparsing, which is what I would use if I were solving a problem similar to this, here is a pyparsing example for the python language.

Like everyone else is saying, your language is confusingly designed and you probably need to simplify it. But I'm going to give you what you're looking for and let you figure that out the hard way.
The standard python file object (returned by open()) is an iterator of lines, and the split() method of the python string class splits a string into a list of substrings. So you'll probably want to start with something like:
for line in command_file
words = line.split(' ')
http://docs.python.org/3/library/string.html

you could use this code to read the file line by line and split it by spaces between words.
a= True
f = open(filename)
while a:
nextline=f.readline()
wordlist= nextline.split("")
print(wordlist)
if nextline=="\n":
a= False

What you're talking about is writing a simple programming language. It's not extraordinarily difficult if you know what you're doing, but it is the sort of thing most people take a full semester class to learn. The fact that you've got multiple different types of lexical units with what looks to be a non-trivial, recursive syntax means that you'll need a scanner and a parser. If you really want to teach yourself to do this, this might not be a bad place to start.
If you simplify your grammar such that each command only has a fixed number of arguments, you can probably get away with using regular expressions to represent the syntax of your individual commands.
Give it a shot. Just don't expect it to all work itself out overnight.

Awk, bash or python for converting a regular file?

I have a text file with lots of lines and with this structure:
[('name_1a',
'name_1b',
value_1),
('name_2a',
'name_2b',
value_2),
.....
.....
('name_XXXa',
'name_XXXb',
value_XXX)]
I would like to convert it to:
name_1a, name_1b, value_1
name_2a, name_2b, value_2
......
name_XXXa, name_XXXb, value_XXX
I wonder what would be the best way, whether awk, python or bash.
Thanks
Jose

Tried evaluating it python? Looks like a list of tuples to me.
eval(your_string)
Note, it's massively unsafe! If there's code in there to delete your hard disk, evaluating it will run that code!

I would like to use Python:
lines = open('filename.txt','r').readlines()
n = len(lines) # n % 3 == 0
for i in range(0,n,3):
name1 = lines[i].strip("',[]\n\r")
name2 = lines[i+1].strip("',[]\n\r")
value = lines[i+2].strip("',[]\n\r")
print name1,name2,value

It looks like legal Python. You might be able to just import it as a module and then write it back out after formatting it.

Oh boy, here is a job for ast.literal_eval:
(literal_eval is safer than eval, since it restricts the input string to literals such as strings, numbers, tuples, lists, dicts, booleans and None:
import ast
filename='in'
with open(filename,'r') as f:
contents=f.read()
data=ast.literal_eval(contents)
for elt in data:
print(', '.join(map(str,elt)))

here's one way to do it with (g)awk
$ awk -vRS=")," ' { gsub(/\n|[\047\]\[)(]/,"") } 1' file
name_1a,name_1b,value_1
name_2a,name_2b,value_2
name_XXXa,name_XXXb,value_XXX

Awk is typically line oriented, and bash is a shell, with limited numbrer of string manipulation functions. It really depends on where your strength as a programmer lies, but all other things being equal, I would choose python.
Did you ever consider that by redirecting the time it took to post this on SO, you could have had it done?
"AWK is a language for processing
files of text. A file is treated as a
sequence of records, and by default
each line is a record. Each line is
broken up into a sequence of fields,
so we can think of the first word in a
line as the first field, the second
word as the second field, and so on.
An AWK program is of a sequence of
pattern-action statements. AWK reads
the input a line at a time. A line is
scanned for each pattern in the
program, and for each pattern that
matches, the associated action is
executed." - Alfred V. Aho[2]

Asking what's the best language for doing a given task is a very different question to say, asking: 'what's the best way of doing a given task in a particular language'. The first, what you're asking, is in most cases entirely subjective.
Since this is a fairly simple task, I would suggest going with what you know (unless you're doing this for learning purposes, which I doubt).
If you know any of the languages you suggested, go ahead and solve this in a matter of minutes. If you know none of them, now enters the subjective part, I would suggest learning Python, since it's so much more fun than the other 2 ;)

If the values are legal python values, you can take advantage of eval() since your data is a legal python data sucture. The following would work if values are integers, otherwise you might have to massage the print call a bit:
input = """[('name_1a',
'name_1b',
1),
('name_2a',
'name_2b',
2),
('name_XXXa',
'name_XXXb',
3)]"""
for e in eval(input):
print '%s,%s,%d' % e
P.S. using eval() is quite controversial since it will execute any valid python code that you pass into it, so take care.