Working with Text Files

In this section we'll cover

Working with f-strings (formatted string literals) to format printed text
Working with Files - opening, reading, writing and appending text files

Formatted String Literals (f-strings)

Introduced in Python 3.6, f-strings offer several benefits over the older .format() string method.
For one, you can bring outside variables immediately into to the string rather than pass them through as keyword arguments:



In [1]:

    
name = 'Fred'

# Using the old .format() method:
print('His name is {var}.'.format(var=name))

# Using f-strings:
print(f'His name is {name}.')









    



His name is Fred.
His name is Fred.

Pass !r to get the string representation:



In [2]:

    
print(f'His name is {name!r}')









    



His name is 'Fred'

Be careful not to let quotation marks in the replacement fields conflict with the quoting used in the outer string:



In [3]:

    
d = {'a':123,'b':456}

print(f'Address: {d['a']} Main Street')









    



  File "<ipython-input-3-b2f08335b9e5>", line 3
    print(f'Address: {d['a']} Main Street')
                         ^
SyntaxError: invalid syntax

Instead, use different styles of quotation marks:



In [4]:

    
d = {'a':123,'b':456}

print(f"Address: {d['a']} Main Street")









    



Address: 123 Main Street

Minimum Widths, Alignment and Padding

You can pass arguments inside a nested set of curly braces to set a minimum width for the field, the alignment and even padding characters.



In [5]:

    
library = [('Author', 'Topic', 'Pages'), ('Twain', 'Rafting', 601), ('Feynman', 'Physics', 95), ('Hamilton', 'Mythology', 144)]

for book in library:
    print(f'{book[0]:{10}} {book[1]:{8}} {book[2]:{7}}')









    



Author     Topic    Pages  
Twain      Rafting      601
Feynman    Physics       95
Hamilton   Mythology     144

Here the first three lines align, except Pages follows a default left-alignment while numbers are right-aligned. Also, the fourth line's page number is pushed to the right as Mythology exceeds the minimum field width of 8. When setting minimum field widths make sure to take the longest item into account.

To set the alignment, use the character < for left-align, ^ for center, > for right.
To set padding, precede the alignment character with the padding character (- and . are common choices).

Let's make some adjustments:



In [6]:

    
for book in library:
    print(f'{book[0]:{10}} {book[1]:{10}} {book[2]:.>{7}}') # here .> was added









    



Author     Topic      ..Pages
Twain      Rafting    ....601
Feynman    Physics    .....95
Hamilton   Mythology  ....144

Date Formatting



In [7]:

    
from datetime import datetime

today = datetime(year=2018, month=1, day=27)

print(f'{today:%B %d, %Y}')









    



January 27, 2018

For more info on formatted string literals visit https://docs.python.org/3/reference/lexical_analysis.html#f-strings

Files

Python uses file objects to interact with external files on your computer. These file objects can be any sort of file you have on your computer, whether it be an audio file, a text file, emails, Excel documents, etc. Note: You will probably need to install certain libraries or modules to interact with those various file types, but they are easily available. (We will cover downloading modules later on in the course).

Python has a built-in open function that allows us to open and play with basic file types. First we will need a file though. We're going to use some IPython magic to create a text file!

Creating a File with IPython

This function is specific to jupyter notebooks! Alternatively, quickly create a simple .txt file with Sublime text editor.



In [8]:

    
%%writefile test.txt
Hello, this is a quick test file.
This is the second line of the file.









    



Overwriting test.txt

Python Opening a File

Know Your File's Location

It's easy to get an error on this step:



In [9]:

    
myfile = open('whoops.txt')









    



---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-9-410403f4f4b4> in <module>()
----> 1 myfile = open('whoops.txt')

FileNotFoundError: [Errno 2] No such file or directory: 'whoops.txt'

To avoid this error, make sure your .txt file is saved in the same location as your notebook. To check your notebook location, use pwd:



In [10]:

    
pwd









    Out[10]:





'C:\\Users\\Mike\\NLP-Bootcamp\\00-Python-Text-Basics'

Alternatively, to grab files from any location on your computer, simply pass in the entire file path.

For Windows you need to use double \ so python doesn't treat the second \ as an escape character, a file path is in the form:

myfile = open("C:\\Users\\YourUserName\\Home\\Folder\\myfile.txt")

For MacOS and Linux you use slashes in the opposite direction:

myfile = open("/Users/YourUserName/Folder/myfile.txt")



In [11]:

    
# Open the text.txt file we created earlier
my_file = open('test.txt')



In [12]:

    
my_file









    Out[12]:





<_io.TextIOWrapper name='test.txt' mode='r' encoding='cp1252'>

my_file is now an open file object held in memory. We'll perform some reading and writing exercises, and then we have to close the file to free up memory.

.read() and .seek()



In [13]:

    
# We can now read the file
my_file.read()









    Out[13]:





'Hello, this is a quick test file.\nThis is the second line of the file.'



In [14]:

    
# But what happens if we try to read it again?
my_file.read()









    Out[14]:





''

This happens because you can imagine the reading "cursor" is at the end of the file after having read it. So there is nothing left to read. We can reset the "cursor" like this:



In [15]:

    
# Seek to the start of file (index 0)
my_file.seek(0)









    Out[15]:





0



In [16]:

    
# Now read again
my_file.read()









    Out[16]:





'Hello, this is a quick test file.\nThis is the second line of the file.'

.readlines()

You can read a file line by line using the readlines method. Use caution with large files, since everything will be held in memory. We will learn how to iterate over large files later in the course.



In [17]:

    
# Readlines returns a list of the lines in the file
my_file.seek(0)
my_file.readlines()









    Out[17]:





['Hello, this is a quick test file.\n', 'This is the second line of the file.']

When you have finished using a file, it is always good practice to close it.



In [18]:

    
my_file.close()

Writing to a File

By default, the open() function will only allow us to read the file. We need to pass the argument 'w' to write over the file. For example:



In [19]:

    
# Add a second argument to the function, 'w' which stands for write.
# Passing 'w+' lets us read and write to the file

my_file = open('test.txt','w+')

**Use caution!**
Opening a file with 'w' or 'w+' *truncates the original*, meaning that anything that was in the original file **is deleted**!



In [20]:

    
# Write to the file
my_file.write('This is a new first line')









    Out[20]:





24



In [21]:

    
# Read the file
my_file.seek(0)
my_file.read()









    Out[21]:





'This is a new first line'



In [22]:

    
my_file.close()  # always do this when you're done with a file

Appending to a File

Passing the argument 'a' opens the file and puts the pointer at the end, so anything written is appended. Like 'w+', 'a+' lets us read and write to a file. If the file does not exist, one will be created.



In [23]:

    
my_file = open('test.txt','a+')
my_file.write('\nThis line is being appended to test.txt')
my_file.write('\nAnd another line here.')









    Out[23]:





23



In [24]:

    
my_file.seek(0)
print(my_file.read())









    



This is a new first line
This line is being appended to test.txt
And another line here.



In [25]:

    
my_file.close()

Appending with `%%writefile`

Jupyter notebook users can do the same thing using IPython cell magic:



In [26]:

    
%%writefile -a test.txt

This is more text being appended to test.txt
And another line here.









    



Appending to test.txt

Add a blank space if you want the first line to begin on its own line, as Jupyter won't recognize escape sequences like \n

Aliases and Context Managers

You can assign temporary variable names as aliases, and manage the opening and closing of files automatically using a context manager:



In [27]:

    
with open('test.txt','r') as txt:
    first_line = txt.readlines()[0]
    
print(first_line)









    



This is a new first line

Note that the with ... as ...: context manager automatically closed test.txt after assigning the first line of text to first_line:



In [28]:

    
txt.read()









    



---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-28-39ca4397fa0a> in <module>()
----> 1 txt.read()

ValueError: I/O operation on closed file.

Iterating through a File



In [29]:

    
with open('test.txt','r') as txt:
    for line in txt:
        print(line, end='')  # the end='' argument removes extra linebreaks









    



This is a new first line
This line is being appended to test.txt
And another line here.
This is more text being appended to test.txt
And another line here.

Great! Now you should be familiar with formatted string literals and working with text files.