Syntax and Usage

Site: Saylor Academy
Course: CS105: Introduction to Python
Book: Syntax and Usage
Printed by: Guest user
Date: Sunday, July 21, 2024, 10:56 PM

Description

Now that you're familiar with file input and output, read this for more on 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/
Creative Commons License 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")

source.txt

1

1.4

0

1.6

-4



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

1

1.4

0

1.6

-4



Methods for reading from a file:


Method Description
open(<filename>) reads everything from a file and reads it as a string
close()
closes an opened file
read() reads everything from a file and reads it as a string
readline() returns a list of strings
readlines() 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)


We can replace the previous program:

mySource = open("source.txt")
for line in mySource:
    print(line)
mySource.close()
source.txt:

1

1.4

0

1.6

-4


with:

mySource = open("source.txt")
text = mySource.read()
prisnant(text)
mySource.close()
 
Python Shell:

1

1.4

0

1.6

-4



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


source.txt

1

1.4

0

1.6

-4



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

1.4

0

1.6

-4


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)


The sum of all values is 0.0.



source.txt

1

1.4

0

1.6

-4


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.


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



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 workWithFile4.py



In-class activity 1

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



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:

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


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

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 


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

1

4

9

16

25

36