starting out with python first edition by tony gaddis
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Chapter 7 Files and Exceptions. STARTING OUT WITH Python First Edition by Tony Gaddis. 7.1 Introduction to File Input and Output. Concept: When a program needs to save data for later use, it writes the data in a file. The data can be read from the file at a later time. - PowerPoint PPT PresentationTRANSCRIPT
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
STARTING OUT WITH
PythonPythonFirst Edition
by Tony Gaddis
Chapter 7Files and Exceptions
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-2
7.1 Introduction to File Input and Output
Concept:
When a program needs to save data for later use, it writes the data in a file. The data can be read from the file at a later time.
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-3
7.1 Introduction to File Input and Output
Terms
• Saving data in a file = “writing data to” the file
• Output file = file that data is written to
• Retrieving data from a file = “reading data from” the file
• Input file = file that data is read from
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7.1 Introduction to File Input and Output
Types of Files
Two types of files:
•Text file - contains data that has been encoded as text, ASCII or Unicode
•Binary file - contains data that has not been converted to text
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7.1 Introduction to File Input and Output
File Access Methods
Two ways to access data stored in files:
•Sequential Access - access data from the beginning of the file to the end of the file
•Direct (random) Access- directly access any piece of data in the file without reading the data that comes before it
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-6
7.1 Introduction to File Input and Output
Opening a File
The open function:
•General format: file_variable = open(filename, mode)
•file_variable is the name of the variable that will reference the file object•filename is a string specifying the name of the file•mode is a string specifying the mode (reading, writing, etc.)
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-7
7.1 Introduction to File Input and Output
customer_file = open(‘customers.txt’, ‘r’)
sales_file = open(‘sales.txt’, ‘w’)
Table 7-1 Some of the Python file modes
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-8
7.1 Introduction to File Input and Output
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-9
7.1 Introduction to File Input and Output
Program 7-1 (file_write.py)
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-10
7.1 Introduction to File Input and Output
Reading Data from a File … read method
Program 7-2 (file_read.py)
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-11
7.1 Introduction to File Input and Output
Reading Data from a File … readline method
Program 7-3 (line_read.py)
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-12
7.1 Introduction to File Input and Output
Concatenating a Newline to a String
myfile.write(name1 + ‘\n’)
Reading a String and Stripping the Newline From It
line1 = infile.readline()
# Strip the \n from the stringline1 = line1.rstrip(‘\n’)
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-13
7.1 Introduction to File Input and Output
Appending Data to an Existing File
•‘a’ mode to open an output file in append mode
•If the file already exists, it will not be erased
•If the file does not exist, it will be created
•When data is written to the file, it will be written at the end of the file’s current contents.
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-14
7.1 Introduction to File Input and Output
Writing and Reading Numeric Data
•Numbers must be converted to strings before they can be written
•Built-in function, str, converts a value to a string
outfile.write(str(num1) + ‘\n’)
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-15
7.1 Introduction to File Input and Output
Writing and Reading Numeric Data
infile = open(‘number.txt’, ‘r’)
# readline method reads strings
string_input = infile.readline()
# built-in function int converts string to an integer
value = int(string_input)
Figure 7-15 The numbers.txt file viewed in Notepad
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-16
7.1 Introduction to File Input and Output
Writing and Reading Numeric Data
Program 7-7 (read_numbers.py)
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7.2 Using Loops to Process Files
Concept:
Files usually hold large amounts of data, and programs typically use a loop to process the data in a file.
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7.2 Using Loops to Process Files
Reading a File with a Loop and Detecting the End of the File
•Read the contents of a file without knowing the number of items that are stored in the file
•readline method returns an empty string (‘ ‘) when it attempts to read beyond the end of file
•Priming read is required to test loop condition, if using a while loop
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-19
7.2 Using Loops to Process Files
Reading a File with a Loop and Detecting the End of the File
Figure 7-17 General logic for detecting the end of a file
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-20
7.2 Using Loops to Process Files
Using Python’s for Loop to Read Lines•for loop automatically reads a line of text from the input file•No special condition or testing is needed•No priming read is needed•Automatically stops when the end of file is reachedfor variable in file_object:
statementstatementetc.
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-21
7.2 Using Loops to Process Files
Using Python’s for Loop to Read Lines
Program 7-10 (read_sales2.py)
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7.3 Processing Records
Concept:The data that is stored in a file is frequently organized in records. A record is a complete set of data about an item, and a field is an individual piece of data within a record.
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7.3 Processing Records
• A file’s data is organized into records and fields• Record – a complete set of data that describes
one item• Field – a single piece of data within a record
Figure 7-19 Records in a file
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7.3 Processing Records
File manipulations
• Create an employee records file
• Read an employee records file
• Add records to a file
• Search for a record in a file
• Modify a record in a file
• Delete a record in a file
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-25
7.3 Processing Records
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7.3 Processing Records
Creating an employee records file … employee.txt
Program 7-13 (save_emp_records.py)
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-27
7.3 Processing Records
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7.3 Processing Records
Reading an employee records file … employee.txt
Program 7-14 (read_emp_records.py)
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7.3 Processing Records
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7.3 Processing Records
Add records to a file … coffee.txt
Program 7-15 (add_coffee_record.py)
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7.3 Processing Records
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7.3 Processing Records
Searching for a record in a file … coffee.txt
Program 7-17 (search_coffee_records.py)
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7.3 Processing Records
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7.3 Processing Records
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7.3 Processing Records
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7.3 Processing Records
Modify a record in a file … coffee.txt
Program 7-18 (modify_coffee_records.py)
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Program 7-18 (cont.) (modify_coffee_records.py)
7.3 Processing Records
Modify a record in a file … coffee.txt
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-38
7.3 Processing Records
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7.3 Processing Records
Deleting a record in a file … coffee.txt
Program 7-19 (delete_coffee_record.py)
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Program 7-19 (cont.) (delete_coffee_record.py)
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7.4 Exceptions
Concept:An exception is an error that occurs while a program is running, causing the program to abruptly halt. You can use the try/exception statement to gracefully handle exceptions.
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Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7-42
7.4 Exceptions
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7.4 Exceptions
Program 7-20(division.py)
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7.4 Exceptions
Program 7-22 (display_file.py)
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7.4 Exceptions
try block
except clause
handler
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7.4 Exceptions
When try/except statement executes, the statements in the try block begin to execute:
•If a statement in the try block raises an exception that is specified by the ExceptionName in an except clause, then the handler that immediately follows the except clause executes. Then, the program resumes execution with the statement immediately following the try/except statement.
•If a statement in the try block raises an exception that is not specified by the ExecptionName in an except clause, then the program will halt with a traceback error message.
•If the statements in the try block execute without raising an exception, then any except clauses and handlers in the statement are skipped and the program resumes execution with the statements immediately following the try/except statement.
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7.4 Exceptions
Figure 7-20 Sequence of events in the try/except statement
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7.4 Exceptions
Program 7-23 (sales_report.py)
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QUESTIONS
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Chapter 7Files and Exceptions