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This article was published as a part of the Data Science Blogathon

Introduction

Python is a simple and easy-to-learn programming language. But still some programmers especially newer ones to the language get mislead. Many programmers have experience with core programming languages like Java, C++, or C#. So they are used to doing it the hard way and when they use this python language which is with easy syntax they misinterpret the diversity and power of Python and sometimes end up surrounded with errors.

👁 Python Mistakes

Source: Google Images

In this article, I will try to put forward those mistakes. This article can be read by any python developer who is at a basic or intermediate level. There are some very common errors like typos, python’s case sensitivity, ignoring Indentation, not using colons for compound statements. These are basic errors that almost everyone is familiar with. We will look at some more errors in detail.

So, let’s get started!

1. Modifying a list while iterating over it

This is a problem faced by almost every python programmer at least once in their coding journey.

Look at this code below:

odd = lambda x : bool(x % 2)
numbers = [n for n in range(20)]
for i in range(len(numbers)):
 if odd(numbers[i]):
 del numbers[i] # Deleting item from a list while iterating over it

Error:

Traceback (most recent call last):
 	 File "<stdin>", line 2, in <module>
IndexError: list index out of range

This problem is obvious but still, sometimes programmers do this. So, the solution to this problem is to use list comprehensions. They are very useful in this kind of situation.

By using list comprehension:

odd = lambda x : bool(x % 2)
numbers = [n for n in range(20)]
numbers[:] = [n for n in numbers if not odd(n)]
print(numbers)

Output:
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

2. Misuse of expressions as defaults for function arguments

This is one of those errors which are hard to debug because it does not give any error and works fine in almost all cases. But this can cause you trouble in complex workspaces.  When we specify a mutable optional function argument this error occurs. Look at the code below to understand better.

def myFunc(myArr=[]):
 myArr.append("Hello")
 return myArr

>>>myFunc()
["Hello"]

>>>myFunc()
["Hello", "Hello"]

>>>myFunc()
["Hello", "Hello", "Hello"]

I hope you got it, this is not the output we were expecting. But since this piece of code doesn’t give any error most of the developers could get away with this if not noticed. Let’s understand this.

In python, the default arguments are evaluated only once. On the first call, it worked fine but on the next calls (second, third) it used the existing myArr instead of initializing it again. So, to solve this:

def myFunc(myArr=None):
 if myArr is None:
 myArr=[]
 myArr.append("Hello")
 return myArr

Output:

>>>myFunc()
["Hello"]
>>>myFunc()
["Hello"]

3. Class variables used incorrectly

Let’s look at this example:

class temp1(obj):
 t=5
class temp2(temp1):
 pass
class temp3(temp1):
 pass
print( temp1.t, temp2.t, temp3.t)

Output

5
5
5

This looks good.

Now if,

temp2.t = 2
print( temp1.t, temp2.t, temp3.t)

Output

5
2
5

This is also fine.

Now if I change the value of class temp1 look what happens:

temp1.t = 3
print( temp1.t, temp2.t, temp3.t)

Output

3
2
3

Boom! Value of class temp3 also changed. But this is not what we wanted. In python, the class variable is internally handled as dictionaries and follows an order known as Method Resolution Order. This happened because variable t was not present in class temp3 so it was looked up in the base class which is temp1. Hence we got this output.

4. Often trying to reinvent the wheel

Programmers coming from low-level language backgrounds like C++, C, etc. try to write everything from scratch. You might be aware that the python community is quite large, here every problem has a solution(seriously). Here we are discussing basic things that python offers. Some of those are related to using of decorators, generators, sort functions, etc. People often write custom sort function which is not needed. It just makes your code length and messy. To make your code more elegant use this:

list.sort(key = , reverse = )

We can sort tuples, dictionaries, or any python object with this. So, it can be used in almost every scenario. Look at these examples:

numbers = [4,5,2,18,3]
numbers.sort(reverse=True)
print("Sorted numbers in Decreasing order:", numbers)

Sorted numbers in Decreasing order: [18, 5, 4, 3, 2]

#take second element for sort
def second(elem):
 return elem[1]

#random list
myList = [('a', 2), ('r', 4), ('t', 1), ('g', 3)]

# sort list with key
myList.sort(key=second)

# print list
print('Sorted :', myList)

Sorted list: [('t', 1), ('a', 2), ('g', 3), ('r', 4)]

assert re.match(VALID_EMAIL_REGEXP, email) is not None

if not re.match(VALID_EMAIL_REGEXP, email):
 raise AssertionError

7.Unnecessary lambda expression

def request(self, method, **kwargs):
 if method not in ("put", "post"):
 req.get_method = lambda: method.upper()

def request(self, method, **kwargs):
 if method not in ("put", "post"):
 req.get_method = method.upper

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