Python working with files

City University of New York (CUNY) City University of New York (CUNY)

CUNY Academic Works CUNY Academic Works

Open Educational Resources Bronx Community College

2019

Python working with 8les Python working with 8les

Natalia Novak

Bronx Community College, City University of New York

How does access to this work bene8t you? Let us know!

More information about this work at: https://academicworks.cuny.edu/bx_oers/25

Discover additional works at: https://academicworks.cuny.edu

This work is made publicly available by the City University of New York (CUNY).

Contact: AcademicWorks@cuny.edu

Files

A common programming task is to read or write information

from/to a file.

Files: reading from files

A common programming task is to read or write information

from/to a file.

Let’s first see how to read from a file:

1) Download the file source.txt from our web-page and

place it into Documents folder

2) type in the following command in the Python Shell:

>>> mySource = open(“source.txt”)

source.txt

1.4

1.6

-4

Files: reading from files

A common programming task is to read or write information

from/to a file.

Let’s first see how to read from a file:

1) Download the file source.txt from our web-page and

place it into Documents folder

2) type in the following command in the Python Shell:

>>> mySource = open(“source.txt”)

Traceback (most recent call last):

File "<pyshell#0>", line 1, in <module>

mySource=open("source.txt")

FileNotFoundError: [Errno 2] No such file or

directory: 'source.txt'

Files: reading from files

A common programming task is to read or write information

from/to a file.

3) Create a new file in Python Editor, type in the following

commands and save the file in the Documents folder

mySource = open(“source.txt”)

for line in mySource:

print(line)

mySource.close()

4) Run it!

1.4

1.6

-4

source.txt :

Files: reading from files

Methods for reading from a file:

method Description

open(<filename>)

opens a file named <filename>

close()

closes an opened file

read()

reads everything from a file and

returns it as string

readlines()

returns a list of strings

readline()

reads one line from a file and returns

it as a string

for i in f

Iterates over the lines in a file (i is a

line from the file)

Files: reading from files

We can replace the previous program:

mySource = open(“source.txt”)

for line in mySource:

print(line)

mySource.close()

with:

mySource = open("source.txt")

text = mySource.read()

print(text)

mySource.close()

1.4

1.6

-4

1.4

1.6

-4

source.txt :

Python Shell:

Files: reading from files

What if we want to find the sum of all numbers in a file?

source.txt

1.4

1.6

-4

Files: reading from files

What if we want to find the sum of all numbers in a file?

mySource = open(“source.txt”)

s = 0

for line in mySource:

s += float(line)

mySource.close()

print(“The sum of all values is”,s)

source.txt

1.4

1.6

-4

Files: reading from files

What if we want to find the sum of all numbers in a file?

mySource = open(“source.txt”)

s = 0

for line in mySource:

s += float(line)

mySource.close()

print(“The sum of all values is”,s)

source.txt

1.4

1.6

-4

You can find this program in the file

workWithFiles3.py

The sum of all values is 0.0

Files: reading from files

What if we want to find the sum of all numbers in a file, but the

source file is formatted differently?

- the numbers are separated by a space or by a new line

source2.txt

1 8 9.2 -5

1.4 9 8

0 12 -23 -9 1

1.6 2.3 -9.1

-42 -91 76 23 7

Files: reading from files

What if we want to find the sum of all numbers in a file, but the

source file is formatted differently?

- the numbers are separated by a space or by a new line

mySource = open("source2.txt")

s = 0

for line in mySource:

line = line.rstrip()

numbers = line.split(" ")

for item in numbers:

s += float(item)

mySource.close()

print("The sum of all values is",s)

source2.txt

1 8 9.2 -5

1.4 9 8

0 12 -23 -9 1

1.6 2.3 -9.1

-42 -91 76 23 7

You can find this program in the file workWithFiles4.py

Files: reading from files

What if we want to find the sum of all numbers in a file, but the

source file is formatted differently?

- the numbers are separated by a comma and by a new line

Modify the program in the file workWithFiles4.py to

accommodate the new formatting.

source3.txt

1,8,9.2,-5

1.4,9,8,13

0,12,-23,-9,1

1.6,2.3,-9.1,-5

-42,-91,7,23,17

In-class activity 1

Files: writing to files

Let’s first see how to write to a file:

Type in the following in the Python Shell:

>>> myOut = open(“out.txt”,’w’)

>>> myOut.write(“Hello, my name is Fran.\n”)

>>> myOut.write(“I like to dance ”)

>>> myOut.write(“and sing.”)

>>> myOut.close()

Let’s try to find the file we just created:

Try the folder where all the programs you worked today with are.

Files: writing to files

Let’s first see how to write to a file:

Type in the following in the Python Shell:

>>> myOut = open(“out.txt”,’w’)

>>> myOut.write(“Hello, my name is Fran.\n”)

>>> myOut.write(“I like to dance ”)

>>> myOut.write(“and sing.”)

>>> myOut.close()

Let’s try to find the file we just created:

Try the folder where all the programs you worked today with are.

Or, do the following:

>>> myOut = open("out.txt")

>>> print(myOut.read())

>>> myOut.close()

Files: writing to files

We can add more to the file that already exists:

Type in the following in the Python Shell:

>>> myOut = open(“out.txt”,’a’)

>>> myOut.write(“I went to the circus today.”)

>>> myOut.close()

Then find the file and see how it changed. Or, do the following:

>>> myOut = open("out.txt")

>>> print(myOut.read())

>>> myOut.close()

You can find this program in the file workWithFiles5.py

Files: writing to files

Methods for writing to a file

method description

open(<filename>,’w’)

open(<filename>,’a’)

open the file for writing (‘w’) or

appending (‘a’)

write(<text>)

writes a string <text> to a file

Files: writing to files

Example: let’s ask the user to enter a positive integer n and a

file name, create a file with the given file name and put the

squares of the first n positive integers into it, separated by

space.

n = int(input("Enter a positive integer:"))

if n > 0:

fname = input("Please enter a file name:")

output = open(fname + ".txt",'w')

for i in range(1,n+1):

output.write(str(i*i)+' ')

output.close()

print("Done! Look for the file",fname+'.txt')

else:

print("positive integer is expected!")

You can find this program in the file workWithFiles6.py

Files: writing to files

In-class activity 2

Modify the program we just did (see file workWithFiles6.py) to

output the squares of the first n positive integers into it,

separated by new line character (‘\n’), i.e. one value per line.

out.txt

Files: writing to files

Proceed to in-class activities 3-4

This OER material was produced as a result of the CS04ALL CUNY OER project.

This work is licensed under a Creative Commons

Attribution-Noncommercial-Share Alike 4.0 License.