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)