Errors and Exceptions

every programmer deals with errors and they can be v. frustrating
understanding what the different error types are and when you are likely to encounter them helps a lot
Errors in python ahve a specific form, called a traceback, let's look



In [8]:

    
cd code









    



/Users/jtdennis/workshops/python-novice-inflammation/code



In [9]:

    
import errors_01



In [10]:

    
errors_01.favorite_ice_cream()









    



---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-10-44b1b870e6a6> in <module>()
----> 1 errors_01.favorite_ice_cream()

/Users/jtdennis/workshops/python-novice-inflammation/code/errors_01.py in favorite_ice_cream()
      5         "strawberry"
      6     ]
----> 7     print(ice_creams[3])

IndexError: list index out of range

Two levels in this traceback:
- the first shows code from teh cell above with an arrow pointing to line 2 (favorite_ice_cream()).
- the second shows somce code in the other function (favorit_ice_cream, located in the file error_01.py on line 7.
- This is where the actual error happened.

LONG tracebacks

sometimes tracebacks are very long (20 levels deep)
this can make it seem like something terrible has happened, but just means your program called many functions before the error
Most times you can just go to the bottom on the trace

What error did we get above?
Last line tells us the type - IndexError and a message "list index out of range"

if you encoutner an error and don't know what it means, read the trace closely
that way if you fix the and then encounter a new one, you can tell that the error change
sometimes just knowing where the error occured is enough to fix it

Don't the error

look at the official documentation on errors on docs.python.org (google it)
it might not be there - it might be a custom error

Syntax Errors

when you forget a colon at the end of a line, add one space too man when indenting uder an if statement, forget parenthesis -- you'll get a syntax errror
this means python couldn't figure out how to read your program
this is similar to forgetting punctuation in English:

this text is difficult to read there is no punctuation there is also no capitalization why is this hard because you have to figure out where each sentence ends you also have to figure out where each sentence begins to some extent it might be ambiguous if there should be a sentence break or not

if python can't figure out how to read a program, it will give up and inform you



In [12]:

    
def some_function()
    msg = "hello, world"
    print(msg)
      return msg









    



  File "<ipython-input-12-3db28f89eee6>", line 1
    def some_function()
                       ^
SyntaxError: invalid syntax

python tells us that there's a syntax error on line 1
event puts a little error where the issue is
we missed the colon at the end of the function definition
let's fix it



In [13]:

    
def some_function():
    msg = "hello, world"
    print(msg)
      return msg









    



  File "<ipython-input-13-65a4c5904cb1>", line 4
    return msg
    ^
IndentationError: unexpected indent

ah, we actually had two errors in that code
now we see an 'IndentationError' and the arrow pointing to the 'return' statement
this tells us or indentation is off on that line

SyntaxError and IndentationError indicate a problem with syntax in your program
IndentationError is more specific - it always means there is aproblem with your indentation in the code

Note on TABS and Spaces

both white spaces
hard to visually tell def.
some editors don't show the diff

Variable Name Errors

very common errors
occurs when you try and use a variable that doesn't exist



In [14]:

    
print(a)









    



---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-14-c5a4f3535135> in <module>()
----> 1 print(a)

NameError: name 'a' is not defined

usually tell us the var name doesn't exist
Why does this occur?
maybe you forgot to quote a string



In [15]:

    
print(hello)









    



---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-15-43a14fcd4265> in <module>()
----> 1 print(hello)

NameError: name 'hello' is not defined

or forgot to create the variable before using it



In [16]:

    
for number in range(10):
    count = count + number
print("The count is:" + str(count))









    



---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-16-0654d41cd0f6> in <module>()
      1 for number in range(10):
----> 2     count = count + number
      3 print("The count is:" + str(count))

NameError: name 'count' is not defined

lastly you might have made a typo
let's say we fixed the above by adding the line Count = 0
you still get an error
remember variables are case sensitive, so the var count is different from Count



In [17]:

    
Count = 0
for number in range(10):
    count = count + number
print("The count is:" + str(count))









    



---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-17-98d5c70d7ddc> in <module>()
      1 Count = 0
      2 for number in range(10):
----> 3     count = count + number
      4 print("The count is:" + str(count))

NameError: name 'count' is not defined

Item errors

errors dealing with containers
trying to access an item in a list that doesn't exist



In [18]:

    
letters = ['a', 'b', 'c']
print("Letter #1 is " + letters[0])
print("Letter #2 is " + letters[1])
print("Letter #3 is " + letters[2])
print("Letter #4 is " + letters[3])









    



Letter #1 is a
Letter #2 is b
Letter #3 is c






    



---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-18-018869cdd091> in <module>()
      3 print("Letter #2 is " + letters[1])
      4 print("Letter #3 is " + letters[2])
----> 5 print("Letter #4 is " + letters[3])

IndexError: list index out of range

IndexError - meaning we tried to access a list index that did not exist

File Errors

errors dealing with reading and writing files



In [19]:

    
file_handle = open('myfile.txt', 'r')









    



---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-19-f6e1ac4aee96> in <module>()
----> 1 file_handle = open('myfile.txt', 'r')

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

ONE reason for reciving this error is that you specified an incorrect path to the file
Eg. i'm in my folder 'python-novice-inflammation' and the file i tried to access is in another folder python-novice-inflammation/writing/myfile.txt, but I tried to open just 'myfile.txt'
the correct path is python-novice-inflammation/writing/myfile.txt
A related issue is we use the read flag instead of the write flag
python will not give an error if you try and open a file for writing
if you meant to open a file for reading, but accidentally opented it for writing, and then try and read from it, pytho will give an UnsupportedOPeration error



In [21]:

    
file_handle = open('myfile', 'w')
file_handle.read()









    



---------------------------------------------------------------------------
UnsupportedOperation                      Traceback (most recent call last)
<ipython-input-21-9d0d96ff7714> in <module>()
      1 file_handle = open('myfile', 'w')
----> 2 file_handle.read()

UnsupportedOperation: not readable



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