Reading and writing files

Why are we so interested in working with files?

As we start this chapter, we find ourselves once again doing things in a slightly different order to most programming books. The majority of introductory programming books won’t consider working with external files until much further along, so why are we introducing it now?

The answer, as was the case in the last chapter, lies in the particular jobs that we want to use Python for. The data that we as biologists work with is stored in files, so if we’re going to write useful programs we need a way to get the data out of files and into our programs (and vice versa). As you were going through the exercises in the previous chapter, it may have occurred to you that copying and pasting a DNA sequence directly into a program each time we want to use it is not a very good approach to take, and you’d be right. The sequences we were working with in the exercises were very short; obviously real-life data will be much longer. Also, it seems inelegant to have the data we want to work on mixed up with the code that manipulates it. In this chapter we’ll see a better way to do it.

We’re lucky in biology that many of the types of data that we work with are stored in text1 files which are relatively simple to process using Python. Chief among these, of course, are DNA and protein sequence data, which can be stored in a variety of formats2 . But there are many other types of data – sequencing reads, quality scores, SNPs, phylogenetic trees, read maps, geographical sample data, genetic distance matrices – which we can access from within our Python programs.

Another reason for our interest in file input/output is the need for our Python programs to work as part of a pipeline or work flow involving other, existing tools. When it comes to using Python in the real world, we often want Python to either accept data from, or provide data to, another program. Often the easiest way to do this is to have Python read, or write, files in a format that the other program already understands.

Reading text from a file

Firstly, a quick note about what we mean by text. In programming, when we talk about text files, we are not necessarily talking about something that is human-readable. Rather, we are talking about a file that contains characters and lines – something that you could open up and view in a text editor, regardless of whether you could actually make sense of the file or not. Examples of text files which you might have encountered include:

  • FASTA files of DNA or protein sequences
  • files containing output from command-line programs (e.g. BLAST)
  • FASTQ files containing DNA sequencing reads
  • HTML files
  • word processing documents
  • and of course, Python code

In contrast, most files that you encounter day-to-day will be binary files – ones which are not made up of characters and lines, but of bytes. Examples include:

  • image files (JPEGs and PNGs)
  • audio files
  • video files
  • compressed files (e.g. ZIP files)

If you’re not sure whether a particular file is text or binary, there’s a very simple way to tell – just open it up in a text editor. If the file displays without any problem, then it’s text (regardless of whether you can make sense of it or not). If you get an error or a warning from your text editor, or the file displays as a collection of indecipherable characters, then it’s binary.

The examples and exercises in this chapter are a little different from those in the previous one, because they rely on the existence of the files that we are going to manipulate. If you want to try running the examples in this chapter, you’ll need to make sure that there is a file in your working directory called dna.txt which has a single line containing a DNA sequence. The easiest way to do this is to run the examples while in the chapter_3 folder inside the exercises download folder.

Using open to read a file

In Python, as in the physical world, we have to open a file before we can read what’s inside it. The Python function that carries out the job of opening a file is very sensibly called open. It takes one argument – a string which contains the name of the file – and returns a file object:

my_file = open("dna.txt")

A file object is a new type which we haven’t encountered before, and it’s a little more complicated than the string and number types that we saw in the previous chapter. With strings and numbers it was easy to understand what they represented – a single bit of text, or a single number. A file object, in contrast, represents something a bit less tangible – it represents a file on your computer’s hard drive.

The way that we use file objects is a bit different to strings and numbers as well. If you glance back at the examples from the previous chapter you’ll see that most of the time when we want to use a variable containing a string or number we just use the variable name:

my_string = 'abcdefg'
my_number = 42
print(my_number + 1)

In contrast, when we’re working with file objects most of our interaction will be through methods. This style of programming will seen unusual at first, but as we’ll see in this chapter, the file type has a well thought-out set of methods which let us do lots of useful things.

The first thing we need to be able to do is to read the contents of the file. The file type has a read method which does this. It doesn’t take any arguments, and the return value is a string, which we can store in a variable. Once we’ve read the file contents into a variable, we can treat them just like any other string – for example, we can print them:

my_file = open("dna.txt")
file_contents =

Files, contents and file names

When learning to work with files it’s very easy to get confused between a file object, a file name, and the contents of a file. Take a look at the following bit of code:

my_file_name = "dna.txt"
my_file = open(my_file_name)
my_file_contents =

What’s going on here? On line 1, we store the string dna.txt in the variable my_file_name. On line 2, we use the variable my_file_name as the argument to the open function, and store the resulting file object in the variable my_file. On line 3, we call the read method on the variable my_file, and store the resulting string in the variable my_file_contents.

The important thing to understand about this code is that there are three separate variables which have different types and which are storing three very different things. my_file_name is a string, and it stores the name of a file on disk. my_file is a file object, and it represents the file itself. my_file_contents is a string, and it stores the text that is in the file.

Remember that variable names are arbitrary – the computer doesn’t care what you call your variables. So this piece of code is exactly the same as the previous example:

apple = "dna.txt"
banana = open(apple)
grape =

except it is harder to read! In contrast, the file name (dna.txt) is not arbitrary – it must correspond to the name of a file on the hard drive of your computer.

A common error is to try to use the read method on the wrong thing. Recall that read is a method that only works on file objects. If we try to use the read method on the file name:

my_file_name = "dna.txt"
my_contents =

we’ll get an AttributeError – Python will complain that strings don’t have a read method3 :

AttributeError: 'str' object has no attribute 'read'

Another common error is to use the file object when we meant to use the file contents. If we try to print the file object:

my_file_name = "dna.txt"
my_file = open(my_file_name)

we won’t get an error, but we’ll get an odd-looking line of output:

<open file 'dna.txt', mode 'r' at 0x7fc5ff7784b0>

We won’t discuss the meaning of this line now: just remember that if you try to print the contents of a file but instead you get some output that looks like the above, you have almost definitely printed the file object rather than the file contents.

Dealing with newlines

Let’s take a look at the output we get when we try to print some information from a file. We’ll use the dna.txt file from the chapter_3 exercises folder. This file contains a single line with a short DNA sequence. Open the file up in a text editor and take a look at it.

We’re going to write a simple program to read the DNA sequence from the file and print it out along with its length. Putting together the file functions and methods from this chapter, and the material we saw in the previous chapter, we get the following code:

# open the file
my_file = open("dna.txt")
# read the contents
my_dna =
# calculate the length
dna_length = len(my_dna)
# print the output
print("sequence is " + my_dna +  " and length is " + str(dna_length))

When we look at the output, we can see that the program is working almost perfectly – but there is something strange: the output has been split over two lines:

 and length is 19

The explanation is simple once you know it: Python has included the new line character at the end of the dna.txt file as part of the contents. In other words, the variable my_dna has a new line character at the end of it. If we could view the my_dna variable directly4 , we would see that it looks like this:


The solution is also simple. Because this is such a common problem, strings have a method for removing new lines from the end of them. The method is called rstrip, and it takes one string argument which is the character that you want to remove. In this case, we want to remove the newline character (\n). Here’s a modified version of the code – note that the argument to rstrip is itself a string so needs to be enclosed in quotes:

my_file = open("dna.txt")
my_file_contents =
# remove the newline from the end of the file contents
my_dna = my_file_contents.rstrip("\n")
dna_length = len(my_dna)
print("sequence is " + my_dna +  " and length is " + str(dna_length))

and now the output looks just as we expected:

sequence is ACTGTACGTGCACTGATC and length is 18

In the code above, we first read the file contents and then removed the newline, in two separate steps:

my_file_contents =
my_dna = my_file_contents.rstrip("\n")

but it’s more common to read the contents and remove the newline all in one go, like this:

my_dna ="\n")

This is a bit tricky to read at first as we are using two different methods (read and rstrip) in the same statement. The key is to read it from left to right – we take the my_file variable and use the read method on it, then we take the output of that method (which we know is a string) and use the rstrip method on it. The result of the rstrip method is then stored in the my_dna variable.

If you find it difficult write the whole thing as one statement like this, just break it up and do the two things separately – your programs will run just as well.

Missing files

What happens if we try to read a file that doesn’t exist?

my_file = open("nonexistent.txt")

We get a new type of error that we’ve not seen before:

IOError: [Errno 2] No such file or directory: 'nonexistent.txt'

Ideally, we’d like to be able to check if a file exists before we try to open it – we’ll see how to do that in section 9. Alternatively, we can deal with missing file errors when they occur: there are many examples in the chapter on exceptions in Advanced Python for Biologists.

Writing text to files

All the example programs that we’ve seen so far in this book have produced output straight to the screen. That’s great for exploring new features and when working on programs, because it allows you to see the effect of changes to the code right away. It has a few drawbacks, however, when writing code that we might want to use in real life.

Printing output to the screen only really works well when there isn’t very much of it5 . It’s great for short programs and status messages, but quickly becomes cumbersome for large amounts of output. Some terminals struggle with large amounts of text, or worse, have a limited scrollback capability which can cause the first bit of your output to disappear. It’s not easy to search in output that’s being displayed at the terminal, and long lines tend to get wrapped. Also, for many programs we want to send different bits of output to different files, rather than having it all dumped in the same place.

Most importantly, terminal output vanishes when you close your terminal program. For small programs like the examples in this book, that’s not a problem – if you want to see the output again you can just re-run the program. If you have a program that requires a few hours to run, that’s not such a great option.

Opening files for writing

In the previous section, we saw how to open a file and read its contents. We can also open a file and write some data to it, but we have to use the open function in a slightly different way. To open a file for writing, we use a two-argument version of the open function, where the second argument is a specially-formatted string describing what we want to do to the file6 . This second argument can be “r” for reading, “w” for writing, or “a” for appending7 . If we leave out the second argument (like we did for all the examples above), Python uses the default, which is “r” for reading.

The difference between “w” and “a” is subtle, but important. If we open a file that already exists using the mode “w”, then we will overwrite the current contents with whatever data we write to it. If we open an existing file with the mode “a”, it will add new data onto the end of the file, but will not remove any existing content. If there doesn’t already exist a file with the specified name, then “w” and “a” behave identically – they will both create a new file to hold the output.

Quite a lot of Python functions and methods have these optional arguments. For the purposes of this book, we will only mention them when they are directly relevant to what we’re doing. If you want to see all the optional arguments for a particular method or function, the best place to look is the official Python documentation – see section 1 for details.

Once we’ve opened a file for writing, we can use the file write method to write some text to it. write works a lot like print – it takes a single string argument – but instead of printing the string to the screen it writes it to the file.

Here’s how we use open with a second argument to open a file and write a single line of text to it:

my_file = open("out.txt", "w")
my_file.write("Hello world")

Because the output is being written to the file in this example, you won’t see any output on the screen if you run it. To check that the code has worked, you’ll have to run it, then open up the file out.txt in your text editor and check that its contents are what you expect8 .

Remember that with write, just like with print, we can use any string as the argument. This also means that we can use any method or function that returns a string. The following are all perfectly OK:

# write "abcdef"
my_file.write("abc" + "def")
# write "8"
# write "TTGC"
my_file.write("ATGC".replace('A', 'T'))
# write "atgc"
# write contents of my_variable

Closing files

There’s one more important file method to look at before we finish this chapter – close. Unsurprisingly, this is the opposite of open (but note that it’s a method, whereas open is a function). We should call close after we’re done reading or writing to a file – we won’t go into the details here, but it’s a good habit to get into as it avoids some types of bugs that can be tricky to track down9 . close is an unusual method as it takes no arguments (so it’s called with an empty pair of parentheses) and doesn’t return any useful value:

my_file = open("out.txt", "w")
my_file.write("Hello world")
# remember to close the file

There’s also way of using a tool called a context manager to take care of closing files automatically: see the exceptions chapter in Advanced Python for Biologists for more details.

Paths and folders

So far, we have only dealt with opening files in the same folder that we are running our program. What if we want to open a file from a different part of the file system?

The open function is quite happy to deal with files from anywhere on your computer, as long as you give the full path (i.e. the sequence of folder names that tells you the location of the file). Just give a file path as the argument rather than a file name. The format of the file path looks different depending on your operating system. If you’re on Linux, it will look like this:

my_file = open("/home/martin/myfolder/myfile.txt")

if you’re on Windows, like this10 :

my_file = open(r"c:\windows\Desktop\myfolder\myfile.txt")

and if you’re on a Mac, like this:

my_file = open("/Users/martin/Desktop/myfolder/myfile.txt")


We’ve taken a whole section to introduce the various ways of reading and writing to files, because it’s such an important part of building programs that are useful in biology. We’ve seen how working with file contents is always a two-step process – we must open a file before reading or writing – and looked at several common pitfalls. We’ll return to the theme of file manipulation in later chapters where we’ll address some of the shortcomings of the techniques we learned in this chapter:

  • All the examples in this chapter than involve reading files do so by reading all the content into a single variable. Often, this is not what we want – a much more common requirement is to process a file line-by-line. In chapter 4 we’ll learn about lists and loops, which will allow us to do exactly that.
  • There are, of course, many things we want to do to files besides simply reading their contents. We would also like our programs to be able to move and copy files, to create and delete files and directories, and to list files and their properties. We’ll cover the tools required to do these things in chapter 9.
  • Finally, another feature common to all our examples is that the names of files are written as part of the code. We will generally want our real-life programs to be more flexible, and capable of reading files that are specified by the user. Chapter 9 also deals with the various forms of user input and in it we’ll learn how to make our programs accept file names flexibly.


Splitting genomic DNA

Look in the chapter_3 folder for a file called genomic_dna.txt – it contains the same piece of genomic DNA that we were using in the final exercise from the previous section. Write a program that will split the genomic DNA into coding and non-coding parts, and write these sequences to two separate files.

Writing a FASTA file

FASTA file format is a commonly-used DNA and protein sequence file format. A single sequence in FASTA format looks like this:


Where sequence_name is a header that describes the sequence (the greater-than symbol indicates the start of the header line). Often, the header contains an accession number that relates to the record for the sequence in a public sequence database. A single FASTA file can contain multiple sequences, like this:


Write a program that will create a FASTA file for the following three sequences – make sure that all sequences are in upper case and only contain the bases A, T, G and C.

Sequence header

DNA sequence







Writing multiple FASTA files

Use the data from the previous exercise, but instead of creating a single FASTA file, create three new FASTA files – one per sequence. The names of the FASTA files should be the same as the sequence header names, with the extension .fasta.


  1. i.e. files which you can open in a text editor and read, as opposed to binary files which cannot be read directly. 

  2. In this course we’ll mostly be talking about FASTA format as it’s the simplest and most common, but there are many more. 

  3. From now on, I’ll just show the relevant bits of output when discussing error message. 

  4. In fact, we can do this – there’s a function called repr that returns a representation of a variable. 

  5. Linux users may be aware that we can redirect terminal output to a file using shell redirection, which can get around some of these problems. 

  6. We call this the mode of the file. 

  7. These are the most commonly-used options – there are a few others. 

  8. .txt is the standard file name extension for a plain text file. Later in this book, when we generate output files with a particular format, we’ll use different file name extensions. 

  9. Specifically, it helps to ensure that output to a file is flushed, which is necessary when we want to make a file available to another program as part of our work flow 

  10. The extra r character before the string is necessary to prevent Python from trying to interpret the backslash in the file path; see section 7 for an explanation. 

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