Python File I/O
Assumed Knowledge
Learning Outcomes
- Be able to open a file for reading
- Be able to write to a file
- Be able to navigate a file structure
Author: Gaurav Gupta
File reading
Reading a file is perhaps the simplest of operations when it comes to File I/O.
There are a couple of ways to open a file for reading:
1. open
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f = open(<file to open>, "r")
<do something with f>
Example:
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f = open("input.xml, "r")
<do something with f>
g = open("C:\Documents\index.html", "r")
<do something with g>
h = open("usr/gauravgupta/grades.csv", "r")
<do something with h>
2. with - as - open
The advantage of opening files in this way is to not have to flush and close them (in case of write mode).
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with open(<file to open>, "r") as f:
<do something with f>
Example:
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with open("input.xml", "r") as f:
<do something with f>
with open("index.html", "r") as g:
<do something with g>
with open("usr/gauravgupta/grades.csv", "r") as h:
<do something with h>
We will continue using the second method from now on.
Reading data from file opened
There are three main functions using which we can read data from files:
read(): returns the contents of entire file as a string.readline(): returns the contents up to a newline character.readlines(): returns a list of different lines in the file.
Say the file contents of file data.txt are:
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This is a sample file.
It contains many sentences.
This being the last one.
read()
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with open("data.txt", "r") as f:
print(f.read())
Output:
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This is a sample file.
It contains many sentences.
This being the last one.
readline()
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with open("data.txt", "r") as f:
print("Line:",f.readline())
print("Line:",f.readline())
print("Line:",f.readline())
Output:
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Line: This is a sample file.
Line: It contains many sentences.
Line: This being the last one.
Reading the whole file line-by-line like this is possible, but awkward.
readlines()
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with open("data.txt", "r") as f:
lines = f.readlines()
for line in lines:
print("Line:",line)
Output:
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Line: This is a sample file.
Line: It contains many sentences.
Line: This being the last one.
Of course, we can cut out the middle-person and reduce it to:
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with open("data.txt", "r") as f:
for line in f.readlines():
print("Line:",line)
Easiest way to read line-by-line
After seeing those three ways, the easiest way to read a file line-by-line is using an iterator, and is given below:
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myfile = open("data.txt", "r")
for line in myfile:
print(line)
Handling headers
You can ignore header line or lines using the next() function.
Example is shown below:
Say the file grades.csv is:
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StudentID, Grade
40404040, 90
41234567, 60
41729000, 70
We would like to ignore the first two lines since the first line is the header and second line is blank.
Output:
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Line: 40404040, 90
Line: 41234567, 60
Line: 41729000, 70
You can store the data in a list or dictionary or whichever collection is most suitable.
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with open('grades.csv', 'r') as f:
records = {}
next(f) # ignore first line
next(f) # ignore second line
for line in f:
line = line.strip()
tokens = line.split(',')
for i in range(len(tokens)):
tokens[i] = int(tokens[i].strip())
# the above processing can also be done using the following:
# tokens = [int(item.strip()) for item in line.strip().split(',')]
records[tokens[0]] = tokens[1] # add to dictionary
print(records)
Output (records):
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{40404040: 90, 41234567: 60, 41729000: 70}
More compact (albeit complex) version of the same program:
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with open('grades_file.csv', 'r') as f:
records = {}
lines = f.readlines()[2:] # skip first two lines
for line in lines:
tokens = [int(item.strip()) for item in line.strip().split(',')]
records[tokens[0]] = tokens[1] # add to dictionary
print(records)
File Writing
You open a file for writing the same way you open it from reading - using the open function, except that you use "w" mode.
Careful! Opening a file in write-mode overwrites the file, without any prompts.
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with open(<file to open for writing>, "w") as f:
<write to file>
Example:
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with open("file.txt", "w") as f:
f.write("Hi!\n")
f.write("Nice to meet you :)\n")
f.write("Bye!\n")
# Can also use:
# f.write("Hi!\nNice to meet you :)\nBye!\n");
We don’t need to explicitly call the close() method. It is handled behind the scenes.
Contents of file.txt after the code is executed:
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Hi!
Nice to meet you :)
Bye!
You can also open a file in append-mode by using "a" as the mode. Assuming file.txt already contains the three lines above, the following code will result in a fourth line appended to it.
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with open("file.txt", "a") as f:
f.write("DONE.\n")
Contents of file.txt after the code is executed:
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Hi!
Nice to meet you :)
Bye!
DONE.
A practical example
Say, we have a csv file that holds student grades. We can read the file, store the data in a collection, perform analysis, and then output filtered information to a file.