16. Python basics: Read and write files

For data analytics we will usually be using two libraries called NumPy and Pandas which have their own simple methods for importing data, but here is the standard Python method. This standard method may sometimes have an advantage that data may be written and read one line at a time without having all data held in memory. This may be useful for very large data files.

File access modes

Files may be opened with different access control models

Text file access modes:

“r” Read (if file does not exist there will be an error)

“w” Write If the file exists contents are overwritten. If the file does not exist, it is created.

“a” Append – will add to file, or will create a file if no file exists

“r+” Read from and write to a file

“w+” Write to and read from a text file. If the file exists contents are overwritten. If the file does not exist, it is created.

“a+” Append and read from a text file. If the file exists, new data is appended to it. If the files doesn’t exist, it’s created.

Creating a file with write

This method where we open a file to save to with the ’w’ argument will create a new file or overwrite an old one.

In this standard way of using a file we begin by opening a file and end by closing the file:

text_file = open('write_it.txt', 'w')
text_file.write('Line 1\n')
text_file.write('This is line 2\n')
text_file.write('That makes this line 3\n')
text_file.close()
\begin{verbatim}

An alternative to opne and closing the file is using the with statement:

\begin{verbatim}
with open('write_it.txt', 'w') as text_file:
    text_file.write('Line 1\n')
    text_file.write('This is line 2\n')
    text_file.write('That makes this line 3\n')

Adding to a file

Using the ’a’ mode we will add to an existing file. We will use the with statement below, but the open/close method will work as well.

with open('write_it.txt', 'a') as text_file:
    text_file.write('This is a fourth line\n')

Writing multiple lines

Multiple lines may be written using a loop with the write method, or may we can use writelines:

'write_it.txt'
text_file = open('write_it.txt', 'w')
lines = ['Line 1\n',
         'This is line 2\n',
         'That makes this line 3\n']
text_file.writelines(lines)
text_file.close()

Reading a file

We can read a file in one line at a time (useful for processing very large files which cannot be held in memory), or we can read in all lines.

Here we read one line at a time:

text_file = open('write_it.txt', 'r')
text_file = open('write_it.txt', 'r')
print(text_file.readline())
print(text_file.readline())
print(text_file.readline())
text_file.close()

OUT:

Line 1

This is line 2

That makes this line 3

Or we can read the entire file in as a list:

text_file = open('write_it.txt', 'r')
lines = text_file.readlines()
text_file.close()
print (lines)

OUT:

['Line 1\n', 'This is line 2\n', 'That makes this line 3\n']
\end{verabtim}

A text file may be stepped through as part of a loop:

\begin{verbatim}
text_file = open('write_it.txt', 'r')
for line in text_file:
    print(line)
text_file.close()
OUT:

Line 1

This is line 2

That makes this line 3

3 thoughts on “16. Python basics: Read and write files

  1. Hey, I don’t typically do a lot of IO, and I found this blog helpful for a little automation script I was working on. In that process though, I found out that if you open a file as “a+,” readlines returns nothing. I suspect it has something to do with the file pointer, but at any rate I was forced to open the file as read-only, close it, and reopen it to append my data. Is there a better way to read and append in one go?

    For context, I was reading a number from the last line, incrementing it, and writing it to the next line.

    Like

    1. Hello Jeremy.

      I don’t go into details of the a+ mode because I tend to prefer to create a new file rather than amend an existing one.

      The reason that readlines is finding nothing is that it is at the end of the file by default. To read from the beginning use my_open_file.seek(0) after opening it. This will read from the beginning (or any given line) but write (append) to the end:

      # Create a file to use for read write in a+ mode
      text_file = open(‘write_it.txt’, ‘w’)
      text_file.write(‘Line 1\n’)
      text_file.write(‘This is line 2\n’)
      text_file.write(‘That makes this line 3\n’)
      text_file.close()

      # open this file in a+ mode (read from start and write to end)
      my_file = open(‘write_it.txt’, ‘a+’)
      my_file.seek(0) # set the read to the start of the file
      # test print first line of file
      print(‘This is the first line of the file:’)
      print(my_file.readline())
      # Append to end of file
      my_file.write(‘This is an appended line\n’)
      my_file.close()

      # Show contents of file after ammending
      print (‘This is the whole of the amended file:’)
      show_file = open(‘write_it.txt’, ‘r’)
      for line in show_file:
      print(line)
      show_file.close()

      Liked by 1 person

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