Errors and Experimentation

Common Errors

Python errors usually come with a message that describes what went wrong. Below are some fo the most common.

SyntaxError

A SyntaxError happens when Python doesn’t understand the code structure (missing a parenthesis, colon, etc.).

print('Hello'

  Cell In[1], line 1
    print('Hello'
                 ^
SyntaxError: incomplete input

print('Hello)

  Cell In[2], line 1
    print('Hello)
          ^
SyntaxError: unterminated string literal (detected at line 1)

NameError

A NameError occurs when you try to use something that hasn’t been defined yet.

print(my_variable)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In[3], line 1
----> 1 print(my_variable)

NameError: name 'my_variable' is not defined

TypeError and ValueError

TypeError means the wrong type of object was used.

ValueError means the type was right but the value didn’t make sense.

"3" + 3

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[4], line 1
----> 1 "3" + 3

TypeError: can only concatenate str (not "int") to str

int("three")

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[5], line 1
----> 1 int("three")

ValueError: invalid literal for int() with base 10: 'three'

IndexError

IndexError means you asked for a position that doesn’t exist.

NOTE: Python allows reverse indexing, so negative index values count back from the end!

numbers = [10, 20, 30]

numbers[5]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[7], line 1
----> 1 numbers[5]

IndexError: list index out of range

numbers[-1]

numbers[-4]

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[9], line 1
----> 1 numbers[-4]

IndexError: list index out of range

File Errors

FileNotFoundError and PermissionError happen when files or paths don’t exist or can’t be accessed. Relative vs. absolute paths, and read/write permissions, can all matter here.

open('missing_file.txt')

---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
Cell In[10], line 1
----> 1 open('missing_file.txt')

File /opt/anaconda3/lib/python3.12/site-packages/IPython/core/interactiveshell.py:324, in _modified_open(file, *args, **kwargs)
    317 if file in {0, 1, 2}:
    318     raise ValueError(
    319         f"IPython won't let you open fd={file} by default "
    320         "as it is likely to crash IPython. If you know what you are doing, "
    321         "you can use builtins' open."
    322     )
--> 324 return io_open(file, *args, **kwargs)

FileNotFoundError: [Errno 2] No such file or directory: 'missing_file.txt'

Experimentation

Resolve issues through debugging and think like like a scientist about to figure out the cause of, and solution, to problems.

Check Types and Values

x = "123"
print(type(x))

<class 'str'>

Print often

a = 5
print("Value of a:", a)

b = 7
print("Value of b:", b)

total = a + b
print("Total is:", total)

total = total + 3
print("Total is:", total)

Value of a: 5
Value of b: 7
Total is: 12
Total is: 15

Check Size and if Empty

empty = []
print(empty)
print(len(empty))

[]
0

not_empty = [1, 2, 3]
print(not_empty)
print(len(not_empty))

[1, 2, 3]
3

Comment and Change Slowly

Use # to comment out lines temporarily. Make one change at a time and re-run to see the effect.

# Swap the commented section
message = "Hello"
print(message)

# numbers = [10, 20, 30]
# print(numbers[2])

Hello

Pro Tip: Use f-Strings

Instead of guessing what a printed value means, f-strings let you add context directly in the output. They are one of the easiest ways to add clarity to print statements and much more.

x = 42
print(f"The value of x is {x}")

name = "Alice"
age = 30
print(f"Name: {name}, Age: {age}")

The value of x is 42
Name: Alice, Age: 30