When coding in Python, you can often anticipate runtime errors even in a syntactically and logically correct program. These errors can be caused by invalid inputs or some predictable inconsistencies.

In Python, you can use the try and the except blocks to handle most of these errors as exceptions all the more gracefully.

In this tutorial, you'll learn the general syntax of try and except. Then we'll proceed to code simple examples, discuss what can go wrong, and provide corrective measures using try and except blocks.

Syntax of Python Try and Except Blocks

Let's start by understanding the syntax of the try and except statements in Python. The general template is shown below:

try:
	# There can be errors in this block
    
except <error type>:
	# Do this to handle exception;
	# executed if the try block throws an error
    
else:
	# Do this if try block executes successfully without errors
   
finally:
	# This block is always executed

Let's look at what the different blocks are used for:

  • The try block is the block of statements you'd like to try executing. However, there may be runtime errors due to an exception, and this block may fail to work as intended.
  • The except block is triggered when the try block fails due to an exception. It contains a set of statements that often give you some context on what went wrong inside the try block.
  • You should always mention the type of error that you intend to catch as exception inside the except block, denoted by the placeholder <error type> in the above snippet.
  • You might as well use except without specifying the <error type>. But, this is not a recommended practice as you're not accounting for the different types of errors that can occur.
In trying to execute the code inside the try block, there's also a possibility for multiple errors to occur.

For example, you may be accessing a list using an index that's way out of range, using a wrong dictionary key, and trying to open a file that does not exist - all inside the try block.

In this case, you may run into IndexError, KeyError, and FileNotFoundError. And you have to add as many except blocks as the number of errors that you anticipate, one for each type of error.

  • The else block is triggered only if the try block is executed without errors. This can be useful when you'd like to take a follow-up action when the try block succeeds. For example, if you try and open a file successfully, you may want to read its content.
  • The finally block is always executed, regardless of what happens in the other blocks. This is useful when you'd like to free up resources after the execution of a particular block of code.
Note: The else and finally blocks are optional. In most cases, you can use only the try block to try doing something, and catch errors as exceptions inside the except block.

Over the next few minutes, you'll use what you've learned thus far to handle exceptions in Python. Let's get started.

How to Handle a ZeroDivisionError in Python

Consider the function divide() shown below. It takes two arguments – num and div – and returns the quotient of the division operation num/div.

def divide(num,div):
  return num/div

▶ Calling the function with different numbers returns results as expected:

res = divide(100,8)
print(res)

# Output
12.5

res = divide(568,64)
print(res)

# Output
8.875

This code works fine until you try dividing by zero:

divide(27,0)

You see that the program crashes throwing a ZeroDivisionError:

# Output
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-19-932ea024ce43> in <module>()
----> 1 divide(27,0)

<ipython-input-1-c98670fd7a12> in divide(num, div)
      1 def divide(num,div):
----> 2   return num/div

ZeroDivisionError: division by zero

You can handle this division by zero as an exception by doing the following:

  • In the try block, place a call to the divide() function. In essence, you're trying to divide num by div.
  • Handle the case when div is 0 as an exception inside the except block.
  • In this example, you can except ZeroDivisionError by printing a message informing the user that they tried dividing by zero.

This is shown in the code snippet below:

try:
    res = divide(num,div)
    print(res)
except ZeroDivisionError:
    print("You tried to divide by zero :( ")

With a valid input, the code still works fine.

divide(10,2)
# Output
5.0

When you try diving by zero, you're notified of the exception that occurs, and the program ends gracefully.

divide(10,0)
# Output
You tried to divide by zero :(

How to Handle a TypeError in Python

In this section, you'll see how you can use try and except to handle a TypeError in Python.

▶ Consider the following function add_10() that takes in a number as the argument, adds 10 to it, and returns the result of this addition.

def add_10(num):
  return num + 10

You can call the function add_10() with any number and it'll work fine, as shown below:

result = add_10(89)
print(result)

#Output
99

Now try calling add_10() with "five" instead of 5.

add_10("five")

You'll notice that your program crashes with the following error message:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-9844e949c84e> in <module>()
----> 1 add_10("five")

<ipython-input-13-2e506d74d919> in add_10(num)
      1 def add_10(num):
----> 2   return num + 10

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

The error message TypeError: can only concatenate str (not "int") to str explains that you can only concatenate two strings, and not add an integer to a string.

Now, you have the following:

  • Given a number my_num, try calling the function add_10() with my_num as the argument. If the argument is of valid type, there's no exception
  • Otherwise, the except block corresponding to the TypeError is triggered, notifying the user that the argument is of invalid type.

This is explained below:

my_num = "five"
try:
  result = add_10(my_num)
  print(result)
except TypeError:
  print("The argument `num` should be a number")

Since you've now handled TypeError as an exception, you're only informed that the argument is of invalid type.

The argument `num` should be a number

How to Handle an IndexError in Python

If you've worked with Python lists, or any Python iterable before, you'll have probably run into IndexError.

This is because it's often difficult to keep track of all changes to iterables. And you may be trying to access an item at an index that's not valid.

▶ In this example, the list my_list has 4 items. The valid indices are 0, 1, 2, and 3, and -1, -2, -3, -4 if you use negative indexing.

As 2 is a valid index, you see that the item at index 2, which is C++, is printed out:

my_list = ["Python","C","C++","JavaScript"]
print(my_list[2])

#Output
C++

If you try accessing an item at index that's outside the range of valid indices, you'll run into an IndexError:

print(my_list[4])
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-7-437bc6501dea> in <module>()
      1 my_list = ["Python","C","C++","JavaScript"]
----> 2 print(my_list[4])

IndexError: list index out of range

If you're familiar with the pattern, you'll now use try and except to handle index errors.

▶ In the code snippet below, you try accessing the item at the index specified by search_idx. 

search_idx = 3
try:
  print(my_list[search_idx])
except IndexError:
  print("Sorry, the list index is out of range")

Here, the search_idx (3) is a valid index, and the item at the particular index is printed out:

JavaScript

If the search_idx is outside the valid range for indices, the except block catches the IndexError as an exception, and there are no more long error messages. 🙂

search_idx = 4
try:
  print(my_list[search_idx])
except IndexError:
  print("Sorry, the list index is out of range")

Rather, the message that the search_idx is out of the valid range of indices is displayed:

Sorry, the list index is out of range

How to Handle a KeyError in Python

You have likely run into KeyError when working with Python dictionaries

▶ Consider this example where you have a dictionary my_dict.

my_dict ={"key1":"value1","key2":"value2","key3":"value3"}
search_key = "non-existent key"
print(my_dict[search_key])
  • The dictionary my_dict has 3 key-value pairs, "key1:value1", "key2:value2", and "key3:value3"
  • Now, you try to tap into the dictionary and access the value corresponding to the key "non-existent key".

As expected, you'll get a KeyError:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-2-2a61d404be04> in <module>()
      1 my_dict ={"key1":"value1","key2":"value2","key3":"value3"}
      2 search_key = "non-existent key"
----> 3 my_dict[search_key]

KeyError: 'non-existent key'

You can handle KeyError in almost the same way you handled IndexError.

  • You can try accessing the value corresponding to the key specified by the search_key.
  • If search_key is indeed a valid key, the corresponding value is printed out.
  • If you run into an exception because of a non-existent key, you use the except block to let the user know.

This is explained in the code snippet below:

try:
  print(my_dict[search_key])
except KeyError:
  print("Sorry, that's not a valid key!")
Sorry, that's not a valid key!

▶ If you want to provide additional context such as the name of the invalid key, you can do that too. It's possible that the key was misspelled which made it invalid. In this case, letting the user know the key used will probably help them fix the typo.

You can do this by catching the invalid key as <error_msg> and use it in the message printed when the exception occurs:

try:
  print(my_dict[search_key])
except KeyError as error_msg:
  print(f"Sorry,{error_msg} is not a valid key!")

▶ Notice how the name of the key is also printed out:

Sorry,'non-existent key' is not a valid key!

How to Handle a FileNotFoundError in Python

Another common error that occurs when working with files in Python is the FileNotFoundError.

▶ In the following example, you're trying to open the file my_file.txt by specifying its path to the function open(). And you'd like to read the file and print out the contents of the file.

However, you haven't yet created the file in the specified location.

If you try running the code snippet below, you'll get a FileNotFoundError:

my_file = open("/content/sample_data/my_file.txt")
contents = my_file.read()
print(contents)
---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-4-4873cac1b11a> in <module>()
----> 1 my_file = open("my_file.txt")

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

And using try and except, you can do the following:

  • Try opening the file in the try block.
  • Handle FileNotFoundError in the except block by letting the user know that they tried to open a file that doesn't exist.
  • If the try block succeeds, and the file does exist, read and print out the contents of the file.
  • In the finally block, close the file so that there's no wastage of resources. Recall how the file will be closed regardless of what happens in the file opening and reading steps.
try:
  my_file = open("/content/sample_data/my_file.txt")
except FileNotFoundError:
  print(f"Sorry, the file does not exist")
else:
  contents = my_file.read()
  print(contents)
finally:
  my_file.close()

Notice how you've handled the error as an exception and the program ends gracefully displaying the message below:

Sorry, the file does not exist

▶ Let's consider the case in which the else block is triggered. The file my_file.txt is now present at the path mentioned earlier.

image-77

And here's what the file my_file.txt contains:

image-78

Now, re-running the earlier code snippet works as expected.

This time, the file my_file.txt is present, the else block is triggered and its contents are printed out, as shown below:

image-80

I hope this clarifies how you can handle exceptions when working with files.

Conclusion

In this tutorial, you've learned how you can use try and except statements in Python to handle exceptions.

You coded examples to understand what types of exception may occur and how you can use except to catch the most common errors.

Hope you enjoyed this tutorial. Happy coding! Until next time :)