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Exceptions and File Input
A201 – Dr. Wennstrom
Tools for Computing: Python
Types of errors
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’
Tools for Computing: Python
Syntax errors
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
SyntaxError: invalid syntax
>>> print 'hello'
SyntaxError: invalid syntax
>>> ls = [4;5;6]
SyntaxError: invalid syntax
>>> for i in range(10):
print(i)
SyntaxError: expected an indented block
Tools for Computing: Python
Erroneous state errors
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'
Tools for Computing: Python
Erroneous state errors
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.
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.
Tools for Computing: Python
Raising an exception
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
>>> while True:
pass
Traceback (most recent call last):
File "<pyshell#53>", line 2, in <module>
pass
KeyboardInterrupt
>>> raise ValueError()
Traceback (most recent call last):
File "<pyshell#54>", line 1, in <module>
raise ValueError()
ValueError
>>> raise ValueError('Just joking...')
Traceback (most recent call last):
File "<pyshell#55>", line 1, in <module>
raise ValueError('Just joking...')
ValueError: Just joking...
>>> try:
raise ValueError()
except:
print('Caught exception.')
Caught exception.
>>>
Tools for Computing: Python
Exception types
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
Tools for Computing: Python
Exceptions
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.
Tools for Computing: Python
Exceptions
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.
Tools for Computing: Python
Do this:
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
Tools for Computing: Python
Catching and handling exceptions
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:= int(input('Enter your age: '))
age
print('You're
age = int(input('Enter
nearly {} years
your
old.'.format(age))
age: '))
print('You're nearly {} years old.'.format(age + 1))
except:
print('Enter your age using digits 0-9!')
The except code
block is the
exception handler
Default behavior:
Custom
behavior:
>>> ==========
========================
RESTART ==========
RESTART ========================
>>>
Enter your age: fifteen
ValueError:
Enter
your age
invalid
usingliteral
digits 0-9!
for int() with base 10: 'fifteen'
Tools for Computing: Python
Format of a try/except statement
The format of a try/except is:
try:
<indented code block>
except:
<exception handler block>
<non-indented statement>
The exception handler processes
any exception raised in the try
block
The except statement is said to
catch the (raised) exception
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>
Tools for Computing: Python
Format of a try/except statement
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
age.txt
default exception
handler prints this
>>> read_age('age.txt')
Value cannot be converted to integer.
>>> read_age('age.text')
IOError: No such file or directory: 'age.text'
Tools for Computing: Python
Multiple exception handlers
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:
It is possible
to restrict the except statement to catch exceptions of
# executed only if a ValueError exception is raised
a specific
type only
print('Value
cannot be converted to integer.')
except:
# executed if an exception other than IOError or ValueError is raised
print('Other error.')
Tools for Computing: Python
Now do this:
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
Tools for Computing: Python
What we really want is to do this:
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
Exceptions and Files
Tools for Computing: Python
What we really want is to do this:
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
Tools for Computing: Python
Files and the file system
root folder
/
Applications
bin
Users
Mail.app
Firefox.app cepope
Contents
image.jpg
MacOS
Mail
binary file
The file system is the OS component
that organizes files and provides a way
to create, access, and modify files
var
Shared
poem.txt
text file
poem.txt
Info
Files are organized in a tree structure
• folders (or directories)
• regular files
The pathname uniquely identifies the file
Relative pathnames (relative to current working directory Users)
Absolute pathnames
• /var/poem.txt
• cepope/poem.txt
• /Users/cepope/poem.txt
• ./cepope/image.jpg
• /Applications/Mail.app/
• ../var/poem.txt
Tools for Computing: Python
Current working directory
>>> import os
>>> os.getcwd()
'/Users'
>>> os.chdir('/Users/cepope')
>>> os.getcwd()
'/Users/cepope'
Tools for Computing: Python
Opening and closing a file
Processing a file consists of:
1. Opening the file
2. Reading from and/or writing to the file
3. Closing the file
Built-in function open() is used to open a file
• The first input argument is the file pathname — may be absolute
or relative to cwd (current working directory)
• The second (optional) argument is the file mode
• Returns a “file” object
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()
Tools for Computing: Python
From now on:
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
Tools for Computing: Python
Do this:
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
Tools for Computing: Python
Improve using try-except:
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
Tools for Computing: Python
with and as:
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
Tools for Computing: Python
Fix up our fileprinter…
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
Moving Through a File
Tools for Computing: Python
Open file mode
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
These are all equivalent
>>>
>>>
>>>
>>>
infile
infile
infile
infile
=
=
=
=
open('poem.txt')
open('poem.txt', 'r')
open('poem.txt', 't')
open('poem.txt', 'rt')
Tools for Computing: Python
File methods
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
Tools for Computing: Python
Reading a file
1 Once upon a midnight dreary, while I pondered weak and weary,\n
⌃ curious volume of forgotten lore,\n
2 Over many a quaint and
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.read(20)
'nce upon a midnight '
>>> infile.readline()
'dreary, while I pondered weak and weary,\n'
>>> s = infile.read()
>>> len(s)
6238
>>> infile.close()
Tools for Computing: Python
Patterns for reading a text file
Common patterns for reading a file:
1. Read the file content into a string
2. Read the file content into a list of words
3. Read the file content into a list of lines
Example:
def word_count(filename):
line_count(filename):
char_count(filename):
'returns the number of lines
characters
words
in file
in filename'
file filename'
infile = open(filename)
open(filename, 'r')
content
lines
= =
infile.readlines()
infile.read()
infile.close()
words = content.split()
return len(lines)
len(content)
return len(words)
Tools for Computing: Python
More file methods
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.read()
''
>>> file.seek(5)
5
>>> file.tell()
5
>>> file.read(7)
'e never'
>>> file.tell()
12
>>> file.seek(0)
0
Tools for Computing: Python
Quiz
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:
while True:
line = file.readline()
if line == "":
break
else:
print(line)
break
except IOError:
print("There was an error opening that file.")