File Input - Computer Science - Lecture Notes, Study notes of Computer Science

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Today

• File Input

Reading

• Ch 15

Homework

• HW 10 due
• HW 11 out

Reading from a file

Programs often take input from a file

• Python provides various functions for working with files:
• The open function opens a file: open(filename)
• Assumes filename is a text file
• Opens it for reading (default)
• Returns a file object (reference)
• If file not in current directory, need to give full path to the file

A simple echo program

• Create a “test.txt” file: hello world spam ham eggs
• Create an “echo.py” script: def main(): the_file = open(’test.txt’) for line in the_file: print(the_line) # prints each line in the_file main()

When this runs ...

• We end up with extra lines
• This is because each line in “test.txt” has a newline (’\n’)

Close files when done with them

• Call the close() method to release resources

the_file = open(’test.txt’) ... do stuff ... the_file.close()

• Once the file is closed ...
• Trying to access its contents will give an error

Each line is considered a string

• What if we wanted to treat the file as a list of numbers?
• Create a file “nums.txt”

10 20 30 40 50 60

• Now convert the strings to numbers:

the_file = open("nums.txt") for line in the_file: num = int(line) print(num)

• Note that in this case, we don’t get the extra lines!
• The int function removes them for us here