Python - Error Handling

# Error Handling

Week 12

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## Try/Except

![](https://media3.giphy.com/media/v1.Y2lkPTc5MGI3NjExdThsejNnaTZ4Mm9nZzZpMTVpejF2cWl5bXg0MnJvdDFseGtmZ3g2aCZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/QMHoU66sBXqqLqYvGO/giphy.webp)

### Handling Errors

```py
n = int( input("Enter a number: ") )
```

What happens if I input `"foobar"` in the terminal?

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

### What Happens?

When an **error** or **exception** are raised, the program stops and displays the exception.

When running a script, the program "crashes" ...

...only if the error is _unhandled_.

NOTES:
- Error and exception are essentially the same in Python.
- In computing, a crash, or system crash, occurs when a computer program such as a software application or an operating system stops functioning properly and exits.

### Introducing Try/Except

```py
try:
  n = int( input("Enter a number: ") )
except:
  print("Sorry, that can't be converted to an integer")
```

Every `try` block must have an `except` block.

### What Could Go Wrong?

```py
def divide_ten():
  string = input("Enter a divisor: ")
  number = int( string )
  dividend = str(10 / number)
  print( "Result: " + dividend )
```

ValueError: invalid literal for int() with base 10

ZeroDivisionError: division by zero

### Handle Specific Errors

```py
try:
  divide_ten()
except ValueError:
  print("ERROR: could not convert input to an integer")
except ZeroDivisionError:
  print("ERROR: divisor must not be zero")
except:
  print("ERROR: something unexpected happened")
```

### Example: Handling Files

```py
try:
  f = open("demofile.txt")
  try:
    f.write("Lorum Ipsum")
  except:
    print("Something went wrong when writing to the file")
  finally:
    f.close()
except:
  print("Something went wrong when opening the file")
```

---

## Else

Use the `else` keyword to define a block of code that is executed _if no errors were raised_.

`else` block must be **after** the `try`/`except` blocks.

`else` block is **optional**.

### Example: Convert a Number

```py
try:
  n = int( input("Enter a number: ") )
except:
  print("Sorry, that can't be converted to an integer")
else:
  print("Good pick, " + str(n) + " is a great number!")
```

---

## Finally

The `finally` block will be executed regardless if the `try` block raises an error or not.

`finally` block must be **after** the `try`/`except`/`else` blocks.

`finally` is **optional**.

### Example: Convert a Number

```py
try:
  n = int( input("Enter a number: ") )
except:
  print("Sorry, that can't be converted to an integer")
else:
  print("Good pick, " + str(n) + " is a great number!")
finally:
  print("All done. Goodbye!")
```

### Example: File Handling

---

## Review

![](https://media1.giphy.com/media/v1.Y2lkPTc5MGI3NjExaDQxcms4anE3dXozcTV6bzlydjkxNGVveWVtcGpwbGZ2dDhtNXg5MiZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/l3vRioHcAYTTZbXQA/giphy.webp)

### Is an Error the Same as an Exception?

Yes! (in Python)

### What is an **Unhandled** Exception?

An exception that has not been handled (doh) and causes the program to crash.

### Are All Bugs Exceptions?

No! Consider the following example:

```py
first = input("First name: ")
last = input("Last name:")
print("Hello, " + first + first + "!")
```

### Do All `try` Blocks Need an `except` Block?

Yes!

### Why Might You Want Multiple `except` Blocks?

To handle different kinds of exceptions.

### What Does the `else` Block Do After `try`/`except`?

Executes code only if the `try` block did not raise an exception.

### What Does the `finally` Block Do?

Executes code regardless of whether an exception was raised.

---

## Exercise

Write a program that does the following:

- Ask user for file name
- Open the file in read (text) mode
- Print the contents of the file
- Use `try`/`except` to prevent crash if the file does not exist