Lesson_05

Lesson 05: File Input and Output

Here are some key notes taken from this lesson which may help with the exercises.

Introduction: Getting Dangerous

Input and output is a core tool in algorithm development and reading from and writing to files is one of the most common forms.

myfile = open('data.txt', 'w')
myfile.write(”I am writing data to my file")
myfile.close()

The variables you put into that open command are the filename(as a string -- do not forget the path) and the file mode. Here we are writing the file, as indicated by the ''w' as the second argument to the open function. This actually returns something called a file object.
Danger: Opening a file that already exists for writing will erase the original file.

There are only a few file modes which we need to use. You have seen 'w' (writing). The others are ''r' (reading), 'a' (appending), 'r+' (reading and writing), and 'b' (binary mode).

A cool way to use contents of a file in a block is with the with command. Formally, this is called a context manager. Informally, it ensures that the file is closed when the block ends.

with open('data.txt') as f:
	print(f.read())

Using with is a good idea but is usually not absolutely necessary. Python tries to close files once they are no longer needed. Having files open is not usually a problem, unless you try to open a large number all at once (e.g. inside a loop).

Reading Lines From Files

Here are some useful methods for file objects:

lines_file = open('fewlines.txt', 'w')
lines_file.writelines("first\n")
lines_file.close()

lines_file = open('fewlines.txt', 'r’)
lines_file.readline()
lines_file.close()

lines = open('fewlines.txt','r').readlines()

Note: both read and readline(s) have optional size arguments that limit how much is read. For readline(s), this may return incomplete lines.

But what if the file is very long and I don't need or want to read all of them at once, file objects behave as their own iterator.

lines_file = open('fewlines.txt','r')
for line in lines_file:
	print(line)

The below syntax is a very common formula for reading through files. Use the with keyword to make sure everything goes smoothly. Loop through the file one line at a time, because often our files have one record to a line. And do something with each line.

with open('fewlines.txt') as myfile:
	for line in my_file:
		print(line.strip()) # The strip function removes newlines and whitespace from the start and finish

Moving Around with tell and seek

The tell method returns the current position of the cursor with in the file. The seek command sets the current position of the cursor within the file.

inputfile = open('data.txt', 'r')
inputfile.tell()
inputfile.read(4)
inputfile.seek()

File-Like Objects

There are other times when you really need to have data in a file (because another function requires it be read from a file perhaps). But why waste time and disk space if you already have the data in memory? A very useful module to make a string into a file-like object is called StringIO. This will take a string and give it file methods like read and write.

import io 
mystringfile = io.StringIO() # For handing bytes, use io.BytesIO
mystringfile.write("This is my data!")#
mystringfile.read()
mystringfile.seek(0)
mystringfile.read()
newstringfile = io.StringIO("My data")

Now let's pretend we have a function that expects to read data from a file before it operates on it. This sometimes happens when using library functions.

def iprintdata(f):
	print(f.read())

iprintdata('my data')
my_io = io.StringIO('my data')
iprintdata(my_io)