Common Mistakes

There are a few common errors and exceptions that you’ll encounter when working with strings and numbers.

strings

Problem: Mismatched string quotes

Mismatched string quotes will result in a SyntaxError

When we try to start a String with one type of quote, and end with another, we’ll see a syntax error.

>>> name = 'Hello"
  File "<stdin>", line 1
    name = "Hello'
                 ^
SyntaxError: EOL while scanning string literal

Solution: use matching quote types for defining your strings. Either single quotes 'Hello' or double quotes "Hello".

Problem: Trying to print a String and a number with concatenation using the “+” symbol.

Trying to add or concatenate a String and a number will result in a TypeError

If you add try to add (or concatenate) a String and a number, you’ll get an error saying that adding the two types together isn’t possible.

>>> print(3 + " Three")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Solutions:

There are two possible solutions here, for two different scenarios.

Converting Types to Strings

In the first scenario, you’d like to add a number to a string via concatenation. In order to do that, you must first convert the number to a string via the str() method. This isn’t so useful while we work in the REPL, but will become useful when we start running Python programs.

>>> my_num = 3
>>> print(str(my_num) + " Three")
3 Three

In the second scenario, you’d like to a convert a number that’s contained in a string (ex: "3") into an Integer, so you can use it like any other number. In this case, you’d like to convert it to an Integer, with the int() method.

>>> str_num = "3"
>>> print(int(str_num) + 5)
8

lists

Problem: Forgetting commas between items

If you forget to include commas between your items, you’ll get a SyntaxError.

>>> numbers = [1, 2 3]
  File "<stdin>", line 1
    numbers = [1, 2 3]
                    ^
SyntaxError: invalid syntax

The REPL makes it difficult to forget the closing bracket, but if you forget it while writing code in a Python file, you’ll see a SyntaxError with a different name. It’ll say: SyntaxError: unexpected EOF while parsing or SyntaxError: invalid syntax.

For example:

# Python file: program.py
names = ["Nina",
x = 5

# In a shell
(env) $ python program.py
  File "/Users/nina/Desktop/program.py", line 2
    x = 5
      ^
SyntaxError: invalid syntax

Notice how the SyntaxError points to a completely valid line of Python code. In these cases, you also need to check the line of code before the line with the SyntaxError. There, we’ll notice that we forgot the closing bracket of our names list.

Indentation

Indentation is important in Python. If you have extra whitespace in your code, you may see an IndentationError.

>>>   a = 5
  File "<stdin>", line 1
    a = 5
IndentationError: unexpected indent

Solution:

Remove the extra whitespace and try again.