Files

📦

import os

cwd = os.getcwd() # store current working dir
print(cwd) # print current working dir
os.chdir('C:\\temp\\python') # change dir
print(os.getcwd()) # print current working dir
os.chdir(cwd) # change working dir back to original
print(os.getcwd())

C:\modules\te3201
C:\temp\python
C:\modules\te3201

💡 Note how the path 'C:\\temp\\python' uses double slash to escape the \. In OS-X or Linux, it can be something like /user/john/python (no need for double slash).

📦 Assume the current working directory is C:\modules\te3201 and you created a new folder inside it named exercises and put a ex.txt file in that folder.

• Absolute path of the file: C:\modules\te3201\exercises\ex1.txt
• Relative path of the file: exercises\ex1.txt

📦 If the current working directory is C:\modules\te3201, you can use any of the following to refer to C:\modules\te3201\exercises\ex1.txt.

• exercises\ex1.txt
• .\exercises\ex1.txt
• ..\te3201\exercises\ex1.txt
• ..\..\modules\te3201\exercises\ex1.txt

📦

print(os.getcwd())
os.makedirs('ex\\w1')
os.chdir('ex\\w1')
print(os.getcwd())
os.chdir('..') # go to parent dir
print(os.getcwd())
os.chdir('..')
os.removedirs('ex\\w1')

C:\repos\nus-te3201\sample-code
C:\repos\nus-te3201\sample-code\ex\w1
C:\repos\nus-te3201\sample-code\ex

📦 Consider the code below:

cwd = os.getcwd()
print(os.path.join(cwd, 'ex', 'w2'))

If you run it on a Windows computer in the folder C:\modules\te3201, it prints C:\modules\te3201\ex\w2.
If your run it on a OS-X computer in the folder /Users/john, it prints /Users/john/ex/w2.

📦 contrasting hard-coding the separator vs using os.path.join():

📦 The code below shows how to read from a text file.

file_path = os.path.join('data', 'items.txt')
f = open(file_path, 'r') # open in read mode
items = f.read()
print(items)
f.close()

first line
second line
third line

💡 The 'r' argument in open(file_path, 'r') indicates that the file should be opened in read mode.

📦 The code below shows how to read file content as a list of lines.

f = open(file_path, 'r')
items = f.readlines()
print(items) # print as a list
for i in items: # print each item
  print(i.strip()) # use strip() to remove linebreak at the end of each line
f.close()

⤵️

['first line\n', 'second line\n', 'third line\n']
first line
second line
third line

💡 Note how each line ends with a \n which represents the line break. It can be removed using the strip() method.

📦 The code below shows how to write to a text file.

file_path = os.path.join('data', 'items.txt')
f = open(file_path, 'w')  # open in write mode
f.write('first line\n')
f.write('second line\n')
f.close()

⤵️ contents of the items.txt:

first line
second line

• 💡 The 'w' argument indicates that the file should be opened in write mode.
• 💡 Unlike the print() function that prints content in a new line every time, the write function does not add an automatic line break at the end. You need to add a \n at each place you want a line break to appear in the file.

📦 The code below shows how to append to a file.

f = open(file_path, 'a')  # open in append mode
f.write('third line\n')
f.close()

⤵️ contents of the items.txt:

first line
second line
third line

📦 Here is the content of a simple CSV file (click here to download a copy) and how it looks like when opened in Excel.

4/11/2017,Alice Bee,4
5/11/2017,Chris Ding,12
5/11/2017,Brenda Chew,13
6/11/2017,Dan Pillai,5

📦 This example shows how to use double quotes to handle commas inside a value:

• 7/11/2017,"Foo, Emily",5 interpreted as three values: 7/11/2017 and Foo, Emily and 5
• 7/11/2017,Foo, Emily,5 interpreted as four values: 7/11/2017 and Foo and Emily and 5

📦 The code below shows how to use the csv module to read contents of a CSV file named deliveries.csv:

deliveries_file = open('deliveries.csv') # open file
deliveries_reader = csv.reader(deliveries_file) # create a Reader
for row in deliveries_reader: # access each line using the Reader
  print(row)
deliveries_file.close() # close file

⤵️

['4/11/2017', 'Alice Bee', '4']
['5/11/2017', 'Chris Ding', '12']
['5/11/2017', 'Brenda Chew', '13']
['6/11/2017', 'Dan Pillai', '5']

As you can see, Reader object returns content of a line as a list object with the value of each cell as an item in the list. Replacing the line,

...
print(row)
...

... with the following line,

...
print('Date:', row[0], '\tRecipient:', row[1], '\tQuantity:', row[2] )
...

... will give you the output shown below:

Date: 4/11/2017 	Recipient: Alice Bee 	Quantity: 4
Date: 5/11/2017 	Recipient: Chris Ding 	Quantity: 12
Date: 5/11/2017 	Recipient: Brenda Chew 	Quantity: 13
Date: 6/11/2017 	Recipient: Dan Pillai 	Quantity: 5

📦 In this example the 3rd value of each row is converted to an int before adding them up.

deliveries_file = open('deliveries.csv') 
deliveries_reader = csv.reader(deliveries_file) 
total = 0
for row in deliveries_reader:
  # convert 3rd cell to an int and add to total
  total = total + int(row[2]) 

print('Total quantity delivered:', total)
deliveries_file.close()

⤵️

Total quantity delivered: 34

📦 The code below writes two rows to the pricelist.csv file.

output_file = open('pricelist.csv', 'w', newline='') # open file in write mode
output_writer = csv.writer(output_file) # get a Writer object
output_writer.writerow(['apples', '1', '1.5', 'True']) # write one row
output_writer.writerow(['bananas', '3', '2.0', 'False']) # write another row
output_file.close() # close file

The pricelist.csv file will now contain:

apples,1,1.5,True
bananas,3,2.0,False

• 💡 You can open a file in append mode if you want to append to it instead of overwriting current content.
  e.g., output_file = open('pricelist.csv','a',newline='')
• 💡 The keyword argument newline='' need to be used when opening a CSV file in Windows. The reasoning behind it is beyond the scope of this book.