Skip to main content

Avoiding Crashes, Try...except ,You are a Software Developer! , Traceback

 

Avoiding Crashes


Sometimes, we just can't code around a crash. It's coming anyway, and all you can do is brace for impact.
Until now!
Let's look at an example based on yesterday's lesson. 


👉 In this example, if the 'Stuff.mine' file doesn't exist, then the code will throw a 'no such file' error.


myStuff = []
f.open("Stuff.mine","r")
myStuff = eval(f.read())
f.close()
for row in myStuff:
  print(row)

Try...except

The new construct to get around this is called try.... except
All the code that should work goes inside the try.


The error messages/instructions to handle any errors running the try code go inside the except

👉 Like this:


myStuff = []

try:

  f.open("Stuff.mine","r")

  myStuff = eval(f.read())

  f.close()

# Try to find a file called 'Stuff.mine' and open it

except:

  print("ERROR: Unable to load")

# If the file can't be found, show the error instead of crashing the whole program

  

for row in myStuff:

  print(row)



You are a Software Developer!


try.... except is great for improving the user experience to reduce frustration.
However, there are problems with just putting the whole code in a 'try except'.
As developers (yes you are a software developer now), it'd be nice to know what sort of error has occurred so that we have a better idea of how to fix it 


We can tell except what type of error(s) to look for. Exception (capital 'E') means 'every type'. I've captured the error type in the 'err' variable and printed it out to tell me what the error is. Here's a list of some built in except error codes


👉 Look at how I've extended the except now.


myStuff = []

try:

  f.open("Stuff.mine","r")

  myStuff = eval(f.read())

  f.close()

# Try to find a file called 'Stuff.mine' and open it

except Exception as err:

  print("ERROR: Unable to load")

  print(err)

  

for row in myStuff:

  print(row)



Traceback

We could even get rid of the 'err' variable entirely and print a traceback, which will show you the red error tracing you see when python crashes.


I've created a 'debugMode' variable at the top of my code and pu the traceback in an if inside the except.


👉 This lets me show/hide the tracebacks easily by setting debugMode to True/False:


debugMode = True

myStuff = []

try:

  f.open("Stuff.mine","r")

  myStuff = eval(f.read())

  f.close()

# Try to find a file called 'Stuff.mine' and open it

except Exception:

  print("ERROR: Unable to load")

  if debugMode:

    print(traceback)

for row in myStuff:

  print(row)








Comments

Post a Comment

Popular posts from this blog

Web Scraping

 Web Scraping Some websites don't have lovely APIs for us to interface with. If we want data from these pages, we have to use a tecnique called scraping. This means downloading the whole webpage and poking at it until we can find the information we want. You're going to use scraping to get the top ten restaurants near you. Get started 👉 Go to a website like Yelp and search for the top 10 reastaurants in your location. Copy the URL.   url = "https://www.yelp.co.uk/search?find_desc=Restaurants&find_loc=San+Francisco%2C+CA%2C+United+States"   Import libraries 👉 Import your libraries. Beautiful soup is a specialist library for extracting the contents of HTML and helping us parse them. Run the Repl once your imports are sorted because we want the Beautiful Soup library to be installed (it'll run quicker this way). import requests from bs4 import BeautifulSoup url = "https://www.yelp.co.uk/search?find_desc=Restaurants&find_loc=San+Francisco%2C+CA%2C+Unite...
Post a Comment
Read more

It's Called Hashing,Hashing, Printing the Hash , Salty, Second User ,

 It's Called Hashing One of the big issues with storing usernames and passwords in a database is what happens if we're hacked? If those passwords are stored as text, our users' security is compromised. Probably across multiple sites because they ignored our advice and used the same password for everything!!!!! Hashing  In reality, organizations don't store your actual password. They store a hash of your password. A hash is produced by turning your password into a sequence of numbers, then passing it though a hashing algorithm (some mathematical process that is very difficult to reverse engineer). The data spit out of this hashing algorithm is what's stored instead of your actual password. 👉 So let's do it. I'm using the built-in hash function to create a numerical hash of the password  password = "baldy1" password = hash(password) print(password) # This will output a really long number  👉 Now let's store that hashed version in our database in...
Post a Comment
Read more

Client/Server Logins

 Client/Server Logins Waaay back when we learned about repl.db, we mentioned the idea of a client/server model for storing data in one place and dishing it out to multiple users. This model is the way we overcome the issue with repl.db of each user getting their own copy of the database. Well, now we can use Flask as a webserver. We can build this client server model to persistently store data in the repl (the server) and have it be accessed by multiple users who access the website via the URL (the clients). Get Started Previously, we have built login systems using Flask & HTML. We're going to start with one of those systems and adapt it to use a dictionary instead. 👉 First, let's remind ourselves of the way the system works. Here's the Flask code. Read the comments for explanations of what it does: from flask import Flask, request, redirect # imports request and redirect as well as flask app = Flask(__name__, static_url_path='/static') # path to the static fil...
Post a Comment
Read more