Common Python Coding Mistakes and How to Fix Them

Here are some of the common mistakes I run into when solving Python code problems.

I'm trying hard to write more bug-free code (if I can).

Common Mistakes

• Typo issue: used a wrong variable.
• Used an improper function name.

ERROR:

if s[left].lower() == s[right].lower:
    left, right = left+1, right-1

OK:

if s[left].lower() == s[right].lower():
    left, right = left+1, right-1

• Initialize an array by reference:

#!/usr/bin/env python
dp = [{}]*(3)
dp[0] = {0:1}

dp[1][1] = 1
print(dp[2][1]) # which one you will get? 1 or None?

dp[1] == dp[2]

• Syntax error for Key data structures.

# set/collection
set1.append() # error
set1.add() # ok

# list
l = l.sort() # unexpected
sorted(l) # in-place change or generate a new list?

# array
array1.push() # ok
array1.pop(1) # ok
array1.pop() # error

# string
str[2:4] = 'ab' # error. Can't change string

• Forget to add the this. decorator for class function members.

#  From:
self.ch = self.string[index]

#  To:
self.ch = self.string[self.index]

#  Errmsg: UnboundLocalError: local variable 'index' referenced before assignment

• array index: For l[left:right], the right element is not excluded.
• Corner case: Forgot to process the initial state.
• Transfer by value vs by reference: Design Twitter

ERROR:

##  Blog link: https://brain.dennyzhang.com/design-twitter
def getNewsFeed(self, userId):
        l = self.tweet_dict[userId]
        for followee in self.follow_dict[userId]:
            l += self.tweet_dict[followee]
        l.sort(reverse=True) 
        return l[0:10]

OK:

##  Blog link: https://brain.dennyzhang.com/design-twitter
def getNewsFeed(self, userId):
        l = copy.deepcopy(self.tweet_dict[userId])
        for followee in self.follow_dict[userId]:
            l += self.tweet_dict[followee]
        l.sort(reverse=True) 
        return l[0:10]

• When adding to a set, we will remove duplicates; meanwhile, when adding to a list, we will keep the duplicates.

ERROR:

##  Blog link: https://brain.dennyzhang.com/design-snake-game
if len(self.foods) == 0 or [x2, y2] != self.foods[0]:
    self.snake_points_set.add((x2, y2))
    self.snake_points.append((x2, y2))

    self.snake_points_set.remove(self.snake_points[0])
    self.snake_points.popleft()
    return self.food_index

OK:

if len(self.foods) == 0 or [x2, y2] != self.foods[0]:
    first_point = self.snake_points.popleft()
    self.snake_points.append((x2, y2))

    self.snake_points_set.add((x2, y2))
    self.snake_points_set.remove(first_point)
    return self.food_index