Syntax and Usage

Files

A common programming task is to read or write information from/to a file.

Source: Natalia Novak, https://academicworks.cuny.edu/bx_oers/25/
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License.

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'

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!>

source.txt

Methods for reading from a file:

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()
prisnant(text)
mySource.close()
 

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

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)
                

mySource = open("source.txt ")                
→ s = 0                
for line in mySource:                
→ s += float(line)                
mySource.close()                
→ print("The sum of all values is" , s)

You can find this program in the file workWithFile3.py

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.

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) 
                    

You can find this program in the file workWithFile4.py

In-class activity 1

Files: Writing to Files

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:

Open the folder containing all the programs you worked on today.

Or, do the following:

>>> myOut = open("out.txt")
>>> print(myOut.read())
>>> myOut.close()

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

Methods for writing to a file

  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 

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.