Syntax and Usage

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