An Introduction To Python - Files, Part 1

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")
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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")
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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 lines
flying
circus
• The first call to readline returns the string "flyingn".
• 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 "circusn".
• 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:
while line != "" :
Process the line.
• The sentinel value is an empty string, which is returned by the readline
method 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 of
its output to start a new line.
• When writing text to an output file, however, you must
explicitly write the newline character to start a new line.
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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: %dnTotal: %8.2fn" %
(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:homeworkinput.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
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What We’ll Be Covering Next Time
1. Files, Part 2
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An Introduction To Python - Files, Part 1

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