An Introduction To Software

Development Using Python

Spring Semester, 2014

Class #19:

Files, Part 1

Data, Data, Data

• In the real world, your program will need to process data. A lot of data.

• In this class we’ve already used two different ways to get data into your programs:– You typed it in (“input”)

– It was given to you in the form of a list (patients)

• Now it’s time to deal with A LOT OF DATA.

• Say hello to files…

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Opening Files: Reading

• To access a file, you must first open it.

• When you open a file, you give the name of the file, or, if the file is stored in a different directory, the file name preceded by the directory path.

• You also specify whether the file is to be opened for reading or writing.

• Suppose you want to read data from a file named input.txt, located in the same directory as the program. Then you use the following function call to open the file:

infile = open("input.txt", "r")Image Credit: www.clipartof.com

Opening Files: Writing

• This statement opens the file for reading (indicated by the string argument "r") and returns a file object that is associated with the file named input.txt.

• The file object returned by the open function must be saved in a variable.

• All operations for accessing a file are made via the file object.

• To open a file for writing, you provide the name of the file as the first argument to the open function and the string "w" as the second argument:

outfile = open("output.txt", "w")Image Credit: www.freepik.com

Closing A File

• If the output file already exists, it is emptied before the new data is written into it.

• If the file does not exist, an empty file is created.

• When you are done processing a file, be sure to close the file using the close method:

infile.close()outfile.close()

• If your program exits without closing a file that was opened for writing, some of the output may not be written to the disk file.

• After a file has been closed, it cannot be used again until it has been reopened.

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Opening / Closing Files Syntax

Reading From A File

• To read a line of text from a file, call the readline method with the file object that was returned when you opened the file:

line = infile.readline()

• When a file is opened, an input marker is positioned at the beginning of the file.

• The readline method reads the text, starting at the current position and continuing until the end of the line is encountered.

• The input marker is then moved to the next line.

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Reading From A File

• The readline method returns the text that it read, including the newline character that denotes the end of the line.

• For example, suppose input.txt contains the linesflyingcircus

• The first call to readline returns the string "flying\n".

• Recall that \n denotes the newline character that indicates the end of the line.

• If you call readline a second time, it returns the string "circus\n".

• Calling readline again yields the empty string "" because you have reached the end of the file. Image Credit: fanart.tv

Blank Lines

• If the file contains a blank line, then readline returns a string containing only the newline character "\n".

• Reading multiple lines of text from a file is very similar to reading a sequence of values with the input function.

• You repeatedly read a line of text and process it until the sentinel value is reached:

line = infile.readline()

while line != "" :Process the line.line = infile.readline()

• The sentinel value is an empty string, which is returned by the readlinemethod after the end of file has been reached.

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What Are You Reading?

• As with the input function, the readline method can only return strings.

• If the file contains numerical data, the strings must be converted to the numerical value using the int or float function:

value = float(line)

• Note that the newline character at the end of the line is ignored when the string is converted to a numerical value.

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Writing To A File

• You can write text to a file that has been opened for writing. This is done by applying the write method to the file object.

• For example, we can write the string "Hello, World!" to our output file using the statement:

outfile.write("Hello, World!\n")

• The print function adds a newline character at the end ofits output to start a new line.

• When writing text to an output file, however, you mustexplicitly write the newline character to start a new line.

Image Credit: olddesignshop.com

Writing To A File

• The write method takes a single string as an argument and writes the string immediately.

• That string is appended to the end of the file, following any text previously written to the file.

• You can also write formatted strings to a file with the write method:outfile.write("Number of entries: %d\nTotal: %8.2f\n" %

(count, total))

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Writing With The Print Statement

• Alternatively, you can write text to a file with the print function.

• Supply the file object as an argument with name file, as follows:

print("Hello, World!", file=outfile)

• If you don’t want a newline, use the end argument:print("Total: ", end="", file=outfile)

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File I/O Example

• Suppose you are given a text file that contains a sequence of floating-point values, stored one value per line. You need to read the values and write them to a new output file, aligned in a column and followed by their total and average value.

32.0

54.0

67.5

80.25

115.0

32.00

54.00

67.50

80.25

115.00

--------

Total: 348.75

Average: 69.75

Input File Output File

One Final Note: Backslashes

• When you specify a file name as a string literal, and the name contains backslash characters (as in a Windows file name), you must supply each backslash twice:

infile = open("c:\\homework\\input.txt", "r")

• A single backslash inside a quoted string is an escape character that is combined with the following character to form a special meaning, such as \n for a newline character.

• The \\ combination denotes a single backslash.

• When supplying a file name to a program, however, a program user should not type the backslash twice.

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What’s In Your Python Toolbox?

print() math strings I/O IF/Else elif While For

Lists And/Or/Not Functions Files

What We Covered Today

1. Opening files

2. Reading from files

3. Writing to files

4. Closing files

Image Credit: http://www.tswdj.com/blog/2011/05/17/the-grooms-checklist/

What We’ll Be Covering Next Time

1. Files, Part 2

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