1. Exceptions and File Input
A201 – Dr. Wennstrom

6. Types of errors There are basically two types of errors: syntax errors
Tools for Computing: Python
There are basically two types of errors:
syntax errors
erroneous state errors
>>> excuse = 'I'm sick'
SyntaxError: invalid syntax
>>> print(hour + ':' + minute + ':' + second)
TypeError: unsupported operand type(s) for +: 'int' and 'str’
>>> infile = open('foo.txt')
IOError: [Errno 2] No such file or directory: 'foo.txt’

7. Syntax errors
Tools for Computing: Python
Syntax errors are due to the incorrect format of a statement
Detected when the statement is translated to machine language, before it is executed.
>>> (3+4]
SyntaxError: invalid syntax
>>> if x == 5
>>> print 'hello'
>>> ls = [4;5;6]
>>> for i in range(10):
print(i)
SyntaxError: expected an indented block

8. Erroneous state errors
Tools for Computing: Python
The program execution gets into an erroneous state during run-time
>>> 3 / 0
ZeroDivisionError: division by zero
>>> ls
NameError: name 'ls' is not defined
>>> ls = [12, 13, 14]
>>> ls[3]
IndexError: list index out of range
>>> ls = ls * ls
TypeError: can't multiply sequence by non-int of type 'list'
>>> int('4.5')
ValueError: invalid literal for int() with base 10: '4.5'

9. Erroneous state errors
Tools for Computing: Python
The program execution gets into an erroneous state during run-time
When a run-time error occurs, an “error” object is created
This object has a type that is related to the type of error
The object contains information about the error
The default behavior is to print this information and interrupt the execution of the statement.
When a run-time error occurs, an “error” object is created
This object has a type that is related to the type of error
The object contains information about the error
The “error” object is called an exception; the creation of an exception due to an error is called the raising of an exception
The programmer can do something special*, using ways to catch an exception and redirect it so as to override the default behavior elsewhere for handling.
*We will see how to do this in a little bit.

10. Raising an exception
Tools for Computing: Python
>>> while True:
pass
Traceback (most recent call last):
File "<pyshell#53>", line 2, in <module>
KeyboardInterrupt
>>> raise ValueError()
File "<pyshell#54>", line 1, in <module>
raise ValueError()
ValueError
>>> raise ValueError('Just joking...')
File "<pyshell#55>", line 1, in <module>
raise ValueError('Just joking...')
ValueError: Just joking...
>>>
>>> while True:
pass
Traceback (most recent call last):
File "<pyshell#53>", line 2, in <module>
KeyboardInterrupt
>>> raise ValueError()
File "<pyshell#54>", line 1, in <module>
raise ValueError()
ValueError
>>> raise ValueError('Just joking...')
File "<pyshell#55>", line 1, in <module>
raise ValueError('Just joking...')
ValueError: Just joking...
>>> try:
except:
print('Caught exception.')
Caught exception.
>>>
>>> while True:
pass
Traceback (most recent call last):
File "<pyshell#53>", line 2, in <module>
KeyboardInterrupt
>>> raise ValueError()
File "<pyshell#54>", line 1, in <module>
raise ValueError()
ValueError
>>>
>>> while True:
pass
>>> while True:
pass
Traceback (most recent call last):
File "<pyshell#53>", line 2, in <module>
KeyboardInterrupt
>>>
By typing Ctrl-C, a user can force a KeyboardInterrupt exception to be raised
Any exception can be raised
within a program with the
raise statement
ValueError, like all exception types, is a class
ValueError() uses the default constructor to create an exception (object)
statement raise switches control flow from normal to exceptional
The constructor can take a “message” argument to be stored in the exception object

11. Exception types
Tools for Computing: Python
Technically an exception is a group of types, just like sequence can be strings, tuples, and lists. Some of the built-in exception classes:
Exception
Explanation
KeyboardInterrupt
Raised when user hits Ctrl-C, the interrupt key
OverflowError
Raised when a floating-point expression evaluates to a value that is too large
ZeroDivisionError
Raised when attempting to divide by 0
IOError
Raised when an I/O operation fails for an I/O-related reason
IndexError
Raised when a sequence index is outside the range of valid indexes
NameError
Raised when attempting to evaluate an unassigned identifier (name)
TypeError
Raised when an operation of function is applied to an object of the wrong type
ValueError
Raised when operation or function has an argument of the right type but incorrect value

12. Exceptions
Tools for Computing: Python
When the program execution gets into an erroneous state, an exception object is created
This object has a type that corresponds to the type of error
The object contains information about the error
The default behavior is to print this information and "crash"
"Exception" does not mean error. It means that the normal execution flow of the program is interrupted and execution switches to the exceptional execution flow.

13. Exceptions
Tools for Computing: Python
The precise categorization of what counts as a ”runtime error" or just an "exception", is different in different programming languages.
We're not going to worry to much about those distinctions in this class. But you do need to know what kinds of situations will cause which type of exception in order to deal with them.
To talk about how we will deal with these exceptions without crashing, let's revisit a problem similar to ones we've had in the past. In fact, that will be our quiz for today.

14. Do this: print("Please enter a number:") user_num = float(input())
Tools for Computing: Python
print("Please enter a number:")
user_num = float(input())
print("The square of " + str(user_num) + " is " + str(user_num**2) + ".")
If we wanted to verify the user's input first using if, we'd need to somehow predict ahead of time whether or not the user's text will successfully convert into a float. Such as….?
The string method .isnumeric() only checks if the numbers are all digits (not decimals or fractions, negative numbers, etc.) You could check all the possibilities ahead of time (.isdigit(), .isdecimal(),…), but that could get complicated.
But Python already knows how to convert strings into floats. It'd be nice if we could just ask the interpreter to try to convert the string into a float, and have it just alert us (instead of crashing) if something goes wrong.
floatingPoint.py

15. Catching and handling exceptions
Tools for Computing: Python
We can override the default handling of a raised exception by using a try/except statement
If an exception is raised during a try block, then the block of the associated except is executed (ie., our contingency plan)
try:
age = int(input('Enter your age: '))
print('You're nearly {} years old.'.format(age + 1))
except:
print('Enter your age using digits 0-9!')
age = int(input('Enter your age: '))
print('You're nearly {} years old.'.format(age))
The except code block is the exception handler
Custom behavior:
Default behavior:
>>> ========== RESTART ==========
>>>
Enter your age: fifteen
Enter your age using digits 0-9!
>>> ======================== RESTART ========================
>>>
Enter your age: fifteen
ValueError: invalid literal for int() with base 10: 'fifteen'

16. Format of a try/except statement
Tools for Computing: Python
The format of a try/except is:
The exception handler processes any exception raised in the try block
The except statement is said to catch the (raised) exception
try:
<indented code block>
except:
<exception handler block>
<non-indented statement>
We can parameterize the except clause to only catch exceptions of
a specific type
try:
<indented code block>
except <ExceptionType>:
<exception handler block>
<non-indented statement>

17. Format of a try/except statement
Tools for Computing: Python
def read_age(filename):
'converts first line of file filename to an integer and prints it'
try:
infile = open(filename)
age = int(infile.readline())
print('age:', age)
except ValueError:
print('Value cannot be converted to integer.')
We can parameterize the except clause to only catch exceptions of
a specific type
1 fifteen
>>> read_age('age.txt')
Value cannot be converted to integer.
>>> read_age('age.txt')
Value cannot be converted to integer.
>>> read_age('age.text')
IOError: No such file or directory: 'age.text'
age.txt
default exception handler prints this

18. Multiple exception handlers
Tools for Computing: Python
def read_age(filename):
'converts first line of file filename to an integer and prints it'
try:
infile = open(filename)
age = int(infile.readline())
print('age:', age)
except IOError:
# executed only if an IOError exception is raised
print('Input/Output error.')
except ValueError:
# executed only if a ValueError exception is raised
print('Value cannot be converted to integer.')
except:
# executed if an exception other than IOError or ValueError is raised
print('Other error.')
It is possible to restrict the except statement to catch exceptions of
a specific type only

19. Now do this: print("Please enter a number:")
Tools for Computing: Python
print("Please enter a number:")
try: user_num = float(input())
print("The square of " + str(user_num) + " is " + str(user_num**2) + ".")
except ValueError:
print("That wasn't a valid number.")
tryExcept.py

20. What we really want is to do this:
Tools for Computing: Python
while True:
print("Please enter a number:")
try: user_input = float(input())
print("The square of " + str(user_input) + " is " \
+ str(user_input**2) + ".")
break
except ValueError:
print("That wasn't a valid number. Please try again.")
tryExceptWhileTrue.py

21. Exceptions and Files

22. What we really want is to do this:
Tools for Computing: Python
There's a whole category of errors that can crop up when working with files.
FileNotFoundError if you try to open a file using a filename that doesn't exist.
IsADirectoryError if you try to open a directory and read it as if it were a text file.
PermissionError if you try to open a file that the current user doesn't have permissions to access. 
Fortunately, all of these error types related to file input and output are subtypes of IOError, so we can account for any kind of problem involving working with files by just catching the IOErrors.
tryExceptWhileTrue.py

23. Files and the file system
Tools for Computing: Python
root folder /
Applications
Users
bin
var
Firefox.app
Mail.app
Shared
cepope
Contents
MacOS
Info
Info /
Applications
Users
bin
var
Firefox.app
Mail.app
Shared
cepope
Contents
MacOS
poem.txt
image.jpg
Mail
The file system is the OS component that organizes files and provides a way to create, access, and modify files
poem.txt
Files are organized in a tree structure
folders (or directories)
folders (or directories)
regular files
binary file
text file
The pathname uniquely identifies the file
Absolute pathnames
/var/poem.txt
/Users/cepope/poem.txt
/Applications/Mail.app/
Relative pathnames (relative to current working directory Users)
cepope/poem.txt
./cepope/image.jpg
../var/poem.txt

24. Current working directory
Tools for Computing: Python
>>> import os
>>> os.getcwd()
'/Users'
>>> os.chdir('/Users/cepope')
'/Users/cepope'
>>> import os
>>> os.getcwd()
'/Users'
>>> os.chdir('/Users/cepope')
>>> import os
>>> os.getcwd()
'/Users'
>>> import os

25. Opening and closing a file
Tools for Computing: Python
Processing a file consists of:
Opening the file
Reading from and/or writing to the file
Closing the file
Built-in function open() is used to open a file
The second (optional) argument is the file mode
Returns a “file” object
The first input argument is the file pathname — may be absolute or relative to cwd (current working directory)
File mode 'r' is used to
open a file for reading
A “file” object is of a type that supports several “file” methods, including close()
>>> infile = open('foo.txt')
IOError: No such file or directory: 'foo.txt'
>>> infile = open('poem.txt', 'r')
>>> infile.close()
>>> infile = open('foo.txt')
IOError: No such file or directory: 'foo.txt'
>>> infile = open('foo.txt')
IOError: No such file or directory: 'foo.txt'
>>> infile = open('poem.txt', 'r')

26. From now on:
Tools for Computing: Python
every command that works with files must be inside of a try block so that you can catch any IOErrors.
Open fileprinter.py from Lab#8
print("Please input a filename.") filename = input("> ") file = open(filename) contents = file.read() print(contents) file.close()
Fileprinter.py

27. Do this: Open fileprinter.py from Lab#8
Tools for Computing: Python
Open fileprinter.py from Lab#8
print("Please input a filename.")
filename = input("> ")
file = open(filename)
contents = file.read()
print(contents)
file.close()
fileprinter.py

28. Improve using try-except:
Tools for Computing: Python
while True:
print("Please input a filename.")
filename = input("> ")
try:
file = open(filename)
contents = file.read()
file.close()
break
except IOError:
print("There was an error opening that file.")
print(contents)
There is one problem with this code. If an error happens after the file is opened but before it is closed, then the file never gets closed!
fileprinter.py

29. with and as:
Tools for Computing: Python
Instead of explicitly closing files using .close(), Python 3 has the keywords with and as which will automatically handle the process of making sure that files are closed for you. Here’s the syntax:
with command-to-create-file as variable-to-store-file:
#code that works with the file object
From now on, you should always use with-as when working with files
fileprinter.py

30. Fix up our fileprinter…
Tools for Computing: Python
while True:
print("Please input a filename.")
filename = input("> ")
try:
with open(filename) as file:
contents = file.read()
break
except IOError:
print("There was an error opening that file.")
print(contents)
fileprinter_try.py

31. Moving Through a File

32. Open file mode The file mode defines how the file will be accessed
Tools for Computing: Python
The file mode defines how the file will be accessed
Mode
Description r
Reading (default) w
Writing (if file exists, content is wiped) a
Append (if file exists, writes are appended) r+
Reading and Writing t
Text (default) b
Binary
>>> infile = open('poem.txt')
>>> infile = open('poem.txt', 'r')
>>> infile = open('poem.txt', 't')
>>> infile = open('poem.txt', 'rt')
These are all equivalent

33. File methods
Tools for Computing: Python
There are several “file” types; they all support similar “file” methods
Methods read() and readline() return the characters read as a string
Methods readlines() returns the characters read as a list of lines
Method write() returns the number of characters written
Usage
Description
infile.read(n)
Read n characters starting from cursor; if fewer than n characters remain, read until the end of file
infile.read()
Read starting from cursor up to the end of the file
infile.readline()
Read starting from cursor up to, and including, the end of line character
infile.readlines()
Read starting from cursor up to the end of the file and return list of lines
outfile.write(s)
Write string s to file outfile starting from cursor
infile.close(n)
Close file infile

34. Reading a file
Tools for Computing: Python
1 Once upon a midnight dreary, while I pondered weak and weary,\n
2 Over many a quaint and curious volume of forgotten lore,\n
3 While I nodded, nearly napping, suddenly there came a tapping\n ⌃
poem.txt
When the file is opened, a cursor is associated with the opened file
The initial position of the cursor is:
under the first character of the file, if file mode is r
beyond the end of the file, if file mode is a or w
>>> infile = open('poem.txt')
>>> infile.read(1)
'O'
>>> infile = open('poem.txt')
>>> infile.read(1)
'O'
>>> infile.read(20)
'nce upon a midnight '
>>> infile.readline()
'dreary, while I pondered weak and weary,\n'
>>> infile = open('poem.txt')
>>> infile.read(1)
'O'
>>> infile.read(20)
'nce upon a midnight '
>>> infile.readline()
'dreary, while I pondered weak and weary,\n'
>>> s = infile.read()
>>> len(s)
6238
>>> infile = open('poem.txt')
>>> infile.read(1)
'O'
>>> infile.read(20)
'nce upon a midnight '
>>> infile.readline()
'dreary, while I pondered weak and weary,\n'
>>> s = infile.read()
>>> len(s)
6238
>>> infile.close()
>>> infile = open('poem.txt')
>>> infile = open('poem.txt')
>>> infile.read(1)
'O'
>>> infile.read(20)
'nce upon a midnight '

35. Patterns for reading a text file
Tools for Computing: Python
Common patterns for reading a file:
Read the file content into a string
Read the file content into a list of words
Read the file content into a list of lines
Example:
def word_count(filename):
'returns the number of words in file filename'
infile = open(filename)
content = infile.read()
infile.close()
words = content.split()
return len(words)
def char_count(filename):
'returns the number of characters in file filename'
infile = open(filename, 'r')
content = infile.read()
infile.close()
return len(content)
def line_count(filename):
'returns the number of lines in file filename'
infile = open(filename)
lines = infile.readlines()
infile.close()
return len(lines)

36. More file methods Example:
Tools for Computing: Python
You can use the method .tell() to figure out exactly where the pointer is, and you can move the pointer to a specific location using .seek().
Example:
>>> file.read()
"never hope to see one.\nBut I can tell you anyhow,\nI'd rather see than be one.\n"
>>> file.read(3)
' '
>>> file.seek(5) 5
>>> file.tell()
>>> file.read(7)
'e never' 12
>>> file.seek(0)

37. Quiz
Tools for Computing: Python
Let's redo our fileprinter.py script so that it reads and prints lines from the file one at a time
Solution:
while True:
print("Please input a filename.")
filename = input("> ")
try:
with open(filename) as file:
line = file.readline()
if line == "":
break
else:
print(line)
except IOError:
print("There was an error opening that file.")