Python: Compound Try/Except Blocks

Question

I am working on a random word picker (used in the command line). The word picker itself works perfectly (I have a text file that has a bunch of words separated by spaces. The program will take the text file and make a list, the entries being everything in the list split with spaces).

The next step I want to take is to be able to programmatically add words to the text file. I was able to get it to add whatever the user gives from input, but now I want to make sure that the word provided from the user doesn’t have spaces, numbers, or uppercase letters.

The first attempt I took looked like this:

def add_word():
     word_to_add = input("Word to add: ")

     valid_word = False
     if any(char.isdigit() for char in word_to_add):
          valid_word = False
     elif any(char.isupper() for char in word_to_add):
          valid word = False
     elif any((" " in chars) for chars in word_to_add):
          valid word = False
     else:
          valid word = True

I know this is a super inefficient method to use. That said, I’m trying to use Try/Except blocks to clean it up a little bit. I think I have an idea of how they work. Since I’m trying to test against multiple conditions, it would be something like

try:
     # code for no numbers
try: 
     # code for no uppercases
try:
     # code for no spaces
except:
     # blah blah blah
else:
     # blah blah blah

Now, the dilemma I’m facing is how do I route each try: to a specific exception? What I mean is if there is an exception with the numbers, it would print something like "your word cannot contain numbers! Try again." and the same deal for spaces and uppercases. Is there a way for me to make something like this:

try:
     # if there's a number go to exception A
try:
     # if there's an uppercase go to exception B
try:
     # if there's a space go to exception C
exception a:
     # blah blah blah
exception b:
     # blah blah blah
exception c:
     # blah blah blah
else:
     # blah blah blah

Would it end up being more efficient to just have multiple try/except blocks or a "compound" version like the above code?

The reason I don’t want to use if/elif/else is because sometimes it will still add the user input to the word list even if valid_word was false.

Sorry that I can’t be more specific here, I just don’t really know how to articulate what I’m trying to accomplish here. Thanks in advance for any help

Asked By: Coder man

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Source

Answer 1

You can raise an exception inside a try block to immediately jump to a matching except, e.g.:

def add_word():
    while True:
        word_to_add = input("Word to add: ")
        try:
            if any(char.isdigit() for char in word_to_add):
                raise ValueError("Your word cannot contain numbers!")
            if any(char.isupper() for char in word_to_add):
                raise ValueError("Your word cannot contain capitals!")
            if " " in word_to_add:
                raise ValueError("Your word cannot contain spaces!")
        except ValueError as err:
            print(err)
            print("Try again")
        else:
            break
    # now do whatever you need to do to add word_to_add

But you could just as easily structure this as an if/elif block where each error condition is an elif and you break the loop in the else:

def add_word():
    while True:
        word_to_add = input("Word to add: ")
        if any(char.isdigit() for char in word_to_add):
            print("Your word cannot contain numbers!")
        elif any(char.isupper() for char in word_to_add):
            print("Your word cannot contain capitals!")
        elif " " in word_to_add:
            print("Your word cannot contain spaces!")
        else:
            break
        print("Try again!")
    # now do whatever you need to do to add word_to_add

Answered By: Samwise

Answer 2

You can only have one try. But it can have multiple excepts. You cannot stack multiple tries. You can legally trys inside each other just like you nest if statements or for loops. But there isn’t much reason to do this in a single function since you can stack multiple excepts on a single try.
The code in the except will only be called when some code raises an error in the try. This most commonly happens when you call a function which then raises an error. But you can also manually raise in the try.

Answered By: Code-Apprentice

Python: Compound Try/Except Blocks

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