Python 10b - Read & Search Files
Reading from a File
To read and print from a file you must open it in read mode by typing "r" instead of "a".
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If you are writing and reading in the same program, make sure you close the file in append mode before you open it in read mode.
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The program below uses the Customers.txt file from the last section.
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A simple for loop can be used to print each line of the file.
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The end = "" code just prevents a space between each line.
Practice Task 1
Open one of the files that you used in Section 10a and print each line.
Example solution:
Reading Specific Lines from a File
Sometimes it is necessary only to print certain lines.
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The following example uses a file where I have written a sentence of advice on each line.
The user is asked to enter a number between 1 and 6.
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If they enter 1, the first line of the file is printed. If they enter 2, the second line of the file is printed etc.
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Remember Python starts counting everything at 0 so each line is a digit less than you would expect.
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Square brackets must be used to denote the line to print: [1] not (1).
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The end = "" code is not necessary but removes space after the line.
Practice Task 2
Create a text file (saved in the same folder as your Python file) with a list of video games.
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Ask the user to enter a number between 1 and 10.
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Print the line for the number that they input.
Example solution:
Print a Random Line
The randint command can be imported and used to print a random line from a file. The example below prints one of the 12 months.
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The file is opened in read mode and the readlines() command extracts each line, placing them in a list.
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A random number is then generated between 0 and one less than the total number of lines.
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This random number is then used to print a line from the extracted list of lines.
Python code
from random import randint
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file = open("months.txt","r")
lines = file.readlines()
random = randint(0,11)
print(lines[random])
months.txt
January
February
March
April
May
June
July
August
September
October
November
December
potential outputs
March
December
April
July
If the length of the file is not fixed (e.g. new lines are being added or lines removed) then the random number generation line can be adapted to use the len command to calculate the current number of lines. This is required to know the upper limit of the random number.
random = randint(0,len(lines)-1)
Searching Through Files
A for loop is used to search through a file, line by line.
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First, an input line is used to allow the user to enter their search term.
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If the term that is being searched for is found, then the whole line is printed.
The example below uses a variable named found to state if there is a match when the file is searched.
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If the search term is found, then the found variable is changed to true.
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If the search term is not found, the found variable remains as false, so the 'no customers found' statement is printed.
Practice Task 3
You should have completed Practice Task 2 from Section 10a (the A Level task).
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Add extra code to that program so that you can search for specific students.
Example solution:
