File Processing

1
File Processing

• CMSC 120 Visualizing Information
• Lecture 4/15/08

2
Files in Python

• Binary
• Data are encoded in binary
• Metadata provides instructions for decoding and
  encoding file
• Plain text
• Data converted to text
• Text converted to numerical representation
• Numerical code saved to file
• One really long string

3
Multi-Line Processing

• ltRETURNgt or ltENTERgt inputs a sequence of
  characters
• end-of-line marker
• newline
• '\n
• gtgtgt print 'Hello\nWorld'
• Hello
• World

4
Escape Sequences

• '\n' newline
• '\t' tab
• '\\' \
• '\'' '
• '\"' "
• '\a' ASCII bell ?
• '\b' ASCII backspace
• '\uXXXX' chr with UNICODE XXXX

5
File Processing

• Open the file
• Associate the file with a name
• Manipulate the file
• Read or Write
• Close the file
• Disassociate from Name

Files are Objects
6
Opening Files

• ltfilenamegt open(ltnamegt, ltmodegt)
• ltnamegt name of the file
• ltmodegt 'r' or 'w'
• gtgtgt infile open('mydata.txt', 'r')

7
Closing Files

• ltfilenamegt.close()
• infile.close()

8
Reading from Files

• Read entire file into a single string
• ltfilenamegt.read()
• Read the next line of the file
• ltfilenamegt.readline()
• Read entire file into a list of lines
• ltfilenamegt.readlines()

9
Reading from a File

• read and display
• first 5 lines of a file
• infile open(someFile, 'r')
• for i in range(5)
• line infile.readline()
• print line-1
• infile.close()

10
Writing to a File

• Just like using the print statement
• ltfilenamegt.write(ltstringgt)
• write two lines to a file
• outfile open('mydata.txt', 'w')
• count 1
• outfile.write('This is the first line\n')
• count count 1
• outfile.write('This is line number d.' count)
• outfile.close()