Handling Files

What will we cover?
  • How to open a file
  • How to read and write to an open file
  • How to close a file.
  • Building an address book
  • Handling binary data files
  • Random Access to file data

Handling files often poses problems for beginners although the reason for this puzzles me slightly. Files in a programming sense are really not very different from files that you use in a word processor or other application: you open them, do some work and then close them again.

The biggest differences are that in a program you access the file sequentially, that is, you read one line at a time starting at the beginning. In practice the word processor often does the same, it just holds the entire file in memory while you work on it and then writes it all back out when you close it. The other difference is that, when programming, you normally open the file as read only or write only. You can write by creating a new file from scratch (or overwriting an existing one) or by appending to an existing one.

One other thing you can do while processing a file is that you can go back to the beginning.

Files - Input and Output

Let's see that in practice. We will assume that a file exists called menu.txt and that it holds a list of meals:

spam & eggs
spam & chips
spam & spam

Now we will write a program to read the file and display the output - like the 'cat' command in Unix or the 'type' command in DOS.

# First open the file to read(r)
inp = file("menu.txt","r")
# read the file into a list then print
# each item
for line in inp.readlines():
    print line
# Now close it again

Note 1: file() takes two arguments. The first is the filename (which may be passed as a variable or a literal string, as we did here). The second is the mode. The mode determines whether we are opening the file for reading(r) or writing(w), and also whether it's for ASCII text or binary usage - by adding a 'b' to the 'r' or 'w', as in: open(fn,"rb")

Note 2: We used the file() function to open the file, older versions of Python used the built in function open() instead. The parameters are identical but since open() is still the preferred mechanism we will usually use open() however, if you find file() more logical then feel free to use that instead.

Note 3: We read and close the file using functions preceded by the file variable. This notation is known as method invocation and is another example of Object Orientation. Don't worry about it for now, except to realize that it's related in some ways to modules. You can think of a file variable as being a reference to a module containing functions that operate on files and which we automatically import every time we create a file type variable.

Note 4: We close the file at the end with the close() method. In Python, files are automatically closed at the end of the program but it is good practice to get into the habit of closing your files explicitly. Why? Well, the operating system may not write the data out to the file until it is closed (this can boost performance). What this means is that if the program exits unexpectedly there is a danger that your precious data may not have been written to the file! So the moral is: once you finish writing to a file, close it.

Note 5: We have not specified the full path to the file in the code above so the file will be treated as being in the current folder. However we can pass a full path name to the file() or open() functions instead of just the file name. There is a wrinkle when using Windows however, because the \ character used to separate folders in a Windows path has a special meaning inside a Python string so when specifying paths in Python it is best to use the / character instead and that will work on any Operating System including Windows.

Now, consider how you could cope with long files. First of all you would need to read the file one line at a time (in Python by using readline() and a while loop instead of readlines() and a for loop). You might then use a line_count variable which is incremented for each line and then tested to see whether it is equal to 25 (for a 25 line screen). If so, you request the user to press a key (enter, say) before resetting line_count to zero and continuing. You might like to try that as an excercise...

Since Python version 2.2 it has also been possible to treat the file as a list so you don't need to use readlines() inside a for loop, you just iterate over the file. Let's rewrite the previous example to see this feature in action:

# First open the file to read(r)
inp = open("menu.txt","r")
# iterate over the file printing each item
for line in inp:
    print line
# Now close it again

This style also has the advantage that there are no limits of memory as with readlines(), so it combines the advantages of a for loop and the while/readline() approach mentioned above.

Really that's all there is to it. You open the file, read it in and manipulate it any way you want to. When you're finished you close the file. However there is one little niggle you may have noticed in the previous example: the lines read from the file have a newline character at the end, so you wind up with blank lines using print (which adds its own newline). To avoid that Python provides a string method called strip() which will remove whitespace, or non-printable characters, from both ends of a string. (It has cousins which can strip one end only called rstrip and lstrip too) If we substitute the print line above with:

for line in inp:
    print line.rstrip()  #only strip right hand end

Everything should now work just fine.

To create a 'copy' command in Python, we simply open a new file in write mode and write the lines to that file instead of printing them. Like this:

# Create the equivalent of: COPY MENU.TXT MENU.BAK

# First open the files to read(r) and write(w)
inp = open("menu.txt","r")
outp = open("menu.bak","w")

# read file, copying each line to new file
for line in inp:

print "1 file copied..."

# Now close the files

Did you notice that I added a print statement at the end, just to reassure the user that something actually happened? This kind of user feedback is usually a good idea.

Because we wrote out the same line that we read in there was no problems with newline characters here. But if we had been writing out strings which we created, or which we had stripped earlier we would have needed to add a newline to the end of the output string, like this:

outp.write(line + '\n') # \n is a newline

Let's look at how we might incorporate that into our copy program. Instead of simply copying the menu we will add todays date to the top. That way we can easily generate a daily menu from the easily modified text file of meals. All we need to do is write out a couple of lines at the top of the new file before copying the menu.txt file, like this:

import time
# Create daily menu based on MENU.TXT
# First open the files to read(r) and write(w)
inp = open("menu.txt","r")
outp = open("menu.prn","w")

# Create todays date string 
today = time.localtime(time.time())
theDate = time.strftime("%A %B %d", today)

# Add Banner text and a blank line
outp.write("Menu for %s\n\n" % theDate) 

# copy each line of menu.txt to new file
for line in inp:

print "Menu created for %s..." % theDate

# Now close the files

Note that we use the time module to get todays date (time.time()) and convert it into a tuple of values (time.localtime()) which are then used by time.strftime() to produce a string which, when inserted into a title message using string formatting, looks like:

Menu for Sunday September 19

Spam & Eggs
Spam &...

Although we added two '\n' characters at the end there is only one blank line printed, that's because one of them is the newline at the end of the title itself. Managing the creation and removal of newline characters is one of the more irritating aspects of handling text files.

Some Operating Systems Gotchas

Operating systems handle files in different ways. This introduces some niggles into our programs if we want them to work on multiple operating systems. There are two niggles in particular which can catch people out and we'll look at them here:


The whole subject of newlines and text files is a murky area of non standard implementation by different operating systems. These differences have their roots in the early days of data communications and the control of mechanical teleprinters. Basically there are 3 different ways to indicate a new line:

  1. A Carriage Return (CR) character ('\r')
  2. A Line Feed (LF) character ('\n')
  3. A CR/LF pair ('\r\n').

All three techniques are used in different operating systems. MS DOS (and therefore Windows) uses method 3. Unix (including Linux) uses method 2. Apple in its original MacOS used method 1, but now uses method 2 since MacOS X is really a variant of Unix.

So how can the poor programmer cope with this multiplicity of line endings? In many languages she just has to do lots of tests and take different action per OS. In more modern languages, including Python, the language provides facilities for dealing with the mess for you. In the case of Python the assistance comes in the form of the os module which defines a variable called linesep which is set to whatever the newline character is on the current operating system. This makes adding newlines easy, and rstrip() takes account of the OS when it does its work of removing them, so really the simple way to stay sane, so far as newlines are concerned is: always use rstrip() to remove newlines from lines read from a file and always add os.linesep to strings being written to a file.

That still leaves the awkward situation where a file is created on one OS and then processed on another, incompatible, OS and sadly, there isn't much we can do about that except to compare the end of the line with os.linesep to determine what the difference is.

Specifying Paths

This is more of an issue for Windows users than others although MacOS 9 users may bump into it occasionally too. As above each OS specifies paths to files using different characters to separate the drives, folders and files. The generic solution for this is again to use the os module which provides the os.sep variable to define the current platforms path separator character. In practice you won't need to use this very often since the path will likely be different for every machine anyway! So instead you will just enter the full path directly in a string, possibly once for each OS you are running on. But there is one big gotcha hiding in wait for Windows users...

You saw in the previous section that python treats the string '\n' as a newline character. That is it takes two characters and treats them as one. In fact there are a whole range of these special sequences beginning with back slash (\) including:

  • \n - A new line
  • \r - A carriage return
  • \t - A horizontal tab
  • \v - A vertical tab
  • \b - A backspace
  • \0nn - Any arbitrary octal character code. e.g. the code \033 is the escape character (ESC)

This means that if we have a data file called test.dat and want to open it in Python by specifying a full Windows path we would expect this to work:

>>> f = open('C:\test.dat')

But Python will see the \t pair as a tab character and complain it cannot find a file called C: est.dat. So how do we get round this inconvenience? There are three solutions:

  1. put an 'r' in front of the string. This tells Python to ignore any back slashes and treat it as a "raw" sting.
    >>> print "Hello\nWorld"
    >>> print r"Hello\nWorld"
  2. Use forward slashes (/) instead of backslashes, Python and Windows will between them sort out the path for you. This has the added advantage of making your code portable to other operating systems too.
  3. Use a double backslash(\\) since a double backslash character is seen by Python as a single backslash!

Thus any of the following will open our data file correctly:

>>> f = open(r'C:\test.dat')
>>> f = open('C:/test.dat')
>>> f = open('C:\\test.dat')

One final twist in file processing is that you might want to append data to the end of an existing file. One way to do that would be to open the file for input, read the data into a list, append the data to the list and then write the whole list out to a new version of the old file. If the file is short that's not a problem but if the file is very large, maybe over 100Mb, then you will simply run out of memory to hold the list. Fortunately there's another mode "a" that we can pass to open() which allows us to append directly to an existing file just by writing. Even better, if the file doesn't exist it will open a new file just as if you'd specified "w".

As an example, let's assume we have a log file that we use for capturing error messages. We don't want to delete the existing messages so we choose to append the error, like this:

def logError(msg):
   err = open("Errors.log","a")

In the real world we would probably want to limit the size of the file in some way. A common technique is to create a filename based on the date, thus when the date changes we automatically create a new file and it is easy for the maintainers of the system to find the errors for a particular day and to archive away old error files if they are not needed. (Remember, from the menu example above, that the time module can be used to find out the current date.)

The Address Book Revisited

You remember the address book program we introduced during the Raw Materials topic and then expanded in the Talking to the User topic? Let's start to make it really useful by saving it to a file and, of course, reading the file at startup. We'll do this by writing some functions. So in this example we pull together several of the strands that we've covered in the last few topics.

The basic design will require a function to read the file at startup, another to write the file at the end of the program. We will also create a function to present the user with a menu of options and a separate function for each menu selection. The menu will allow the user to:

Loading the Address Book

filename = "addbook.dat"

def readBook(book):
    import os
    if os.path.exists(filename):
       store = open(filename,'r')
       for line in store:
          name = line.rstrip()
          entry = store.next().rstrip()
          book[name] = entry

Notice the use of rstrip() to remove the new-line character from the end of the line. Also notice the next() operation to fetch the next line from the file within the loop. Finally notice that we defined the filename as a module level variable so we can use it both in loading and saving the data.

Saving the Address Book

def saveBook(book):
    store = open(filename, 'w')
    for name,entry in book.items():
        store.write(name + '\n')
        store.write(entry + '\n')

Notice we need to add a newline character ('\n') when we write the data.

Getting User Input

def getChoice(menu):
    print menu
    choice = int( raw_input("Select a choice(1-4): ") )
    return choice

Adding an Entry

def addEntry(book):
    name = raw_input("Enter a name: ")
    entry = raw_input("Enter street, town and phone number: ")
    book[name] = entry

Removing an entry

def removeEntry(book):
    name = raw_input("Enter a name: ")

Finding an entry

def findEntry(book):
    name = raw_input("Enter a name: ")
    if name in book:
       print name, book[name]
    else: print "Sorry, no entry for: ", name

Quitting the program

Actually I won't write a separate function for this, instead I'll make the quit option the test in my menu while loop. So the main program will look like this:

def main():
    theMenu = '''
    1) Add Entry
    2) Remove Entry
    3) Find Entry
    4) Quit and save
    theBook = {}
    choice = getChoice(theMenu)
    while choice != 4:
        if choice == 1:
        elif choice == 2:
        elif choice == 3:
        else: print "Invalid choice, try again"
        choice = getChoice(theMenu)

Now the only thing left to do is call the main() function when the program is run, and to do that we use a bit of Python magic like this:

if __name__ == "__main__":

This mysterious bit of code allows us to use any python file as a module by importing it, or as a program by running it. The difference is that when the program is imported, the internal variable __name__ is set to the module name but when the file is run, the value of __name__ is set to "__main__". Sneaky, eh?

Now if you type all that code into a new text file and save it as addressbook.py, you should be able to run it from an OS prompt by typing:

C:\PROJECTS> python addressbook.py

Or just double click the file in Windows Explorer and it should start up in its own DOS window, and the window will close when you select the quit option.

Or in Linux:

$ python addressbook.py

Study the code, see if you can find the mistakes (I've left, at least, two minor bugs for you to find, there may be more!) and try to fix them. This 60 odd line program is typical of the sort of thing you can start writing for yourself. There are a couple of things we can do to improve it which I'll cover in the next section, but even as it stands it's a reasonably useful little tool.

VBScript and JavaScript

Neither VBScript nor JavaScript have native file handling capabilities. This is a security feature to ensure nobody can read your files when you innocently load a web page, but it does restrict their general usefulness. However, as we saw with reusable modules there is a way to do it using Windows Script Host. WSH provides a FileSystem object which allows any WSH language to read files. We will look at a JavaScript example in detail then show similar code in VBScript for comparison, but as before the key elements will really be calls to the WScript objects.

Before we can look at the code in detail it's worth taking time to describe the FileSystem Object Model. An Object Model is a set of related objects which can be used by the programmer. The WSH FileSystem object model consists of the FSO object, a number of File objects, including the TextFile object which we will use. There are also some helper objects, most notable of which is, for our purposes, the TextStream object. Basically we will create an instance of the FSO object, then use it to create our TextFile objects and from these in turn create TextStream objects to which we can read or write text. The TextStream objects themselves are what we actually read/write from the files.

Type the following code into a file called testFiles.js and run it using cscript as described in the earlier introduction to WSH.

Opening a file

To open a file in WSH we create an FSO object then create a TextFile object from that:

var fileName, fso, txtFile, outFile, line;

// Get file name
fso = new ActiveXObject("Scripting.FileSystemObject");
WScript.Echo("What file name? ");
fileName = WScript.StdIn.Readline();

// open inFile to read, outFile to write
inFile = fso.OpenTextFile(fileName, 1); // mode 1 = Read
fileName = fileName + ".BAK"
outFile = fso.CreateTextFile(fileName);

Reading and Writing a file

// loop over file till it reaches the end
while ( !inFile.AtEndOfStream ){
    line = inFile.ReadLine();
    outFile.WriteLine( line );

Closing files


And in VBScript

Save the following as testFIles.ws and then run it using:

cscript testfiles.ws

Or alternatively, put the bit between the script tags into a file called testFile.vbs and run that instead. The .ws format allows you to mix JavaScript and VBScript code in the same file by simply using multiple script tags, should you want to...

<?xml version="1.0"?>

  <script type="text/vbscript">
      Dim fso, inFile, outFile, inFileName, outFileName
      Set fso = CreateObject("Scripting.FileSystemObject")
      WScript.Echo "Type a filename to backup"
      inFileName = WScript.StdIn.ReadLine
      outFileName = inFileName &amp; ".BAK"
      ' open the files
      Set inFile = fso.OpenTextFile(inFileName, 1)
      Set outFile = fso.CreateTextFile(outFileName)

      ' read the file and write to the backup copy
      While not inFile.AtEndOfStream
         line = inFile.ReadLine
      ' close both files
      WScript.Echo inFileName &amp; " backed up to " &amp; outFileName

Handling Non-Text Files

Handling text is one of the most common things that programmers do, but sometimes we need to process raw binary data too. This is very rarely done in VBScript or JavaScript so I will only be covering how Python does it.

Opening and Closing Binary Files

The key difference between text files and binary files is that text files are composed of octets, or bytes, of binary data whereby each byte represents a character and the end of the file is marked by a special byte pattern, known generically as end of file, or eof. A binary file contains arbitrary binary data and thus no specific value can be used to identify end of file, thus a different mode of operation is required to read these files. The end result of this is that when we open a binary file in Python (or indeed any other language) we must specify that it is being opened in binary mode or risk the data being read being truncated at the first eof character that Python finds in the data. The way we do this in Python is to add a 'b' to the mode parameter, like this:

binfile = file("aBinaryFile.bin","rb")

The only difference from opening a text file is the mode value of "rb". You can use any of the other modes too, simply add a 'b': "wb" to write, "ab" to append.

Closing a binary file is no different to a text file, simply call the close() method of the open file object:


Because the file was opened in binary mode there is no need to give Python any extra information, it knows how to close the file correctly.

Data Representation and Storage

Before we discuss how to access the data within a binary file we need to consider how data is represented and stored on a computer. All data is stored as a sequence of binary digits, or bits. These bits are grouped into sets of 8 or 16 called bytes or words respectively. (A group of 4 is sometimes called a nibble!) A byte can be any one of 256 different bit patterns and these are given the values 0-255.

The information we manipulate in our programs, strings, numbers etc must all be converted into sequences of bytes. Thus the characters that we use in strings are each allocated a particular byte pattern. There were originally several such encodings, but the most common is the ASCII (American Standard Coding for Information Interchange). Unfortunately pure ASCII only caters for 128 values which is not enough for non English languages. A new encoding standard known as Unicode has been produced, which can use data words instead of bytes to represent characters, and allows for over 65000 characters. A subset of Unicode called UTF-8 corresponds closely to the earlier ASCII coding such that every valid ASCII file is a valid UTF-8 file, although not necessarily the other way around. Python by default supports ASCII and by pre-pending a u in front of a string we can tell Python to treat the string as Unicode. (Although we also need to tell Python which encoding is in use or it will be confused.)

In the same way numbers need to be converted to binary codings too. For small integers it is simple enough to use the byte values directly, but for numbers larger than 255 (or negative numbers, or fractions) some additional work needs to be done. Over time various standard codings have emerged for numerical data and most programming languages and operating systems use these. For example, the American Institute of Electrical and Electronic Engineering (IEEE) have defined a number of codings for floating point numbers.

The point of all of this is that when we read a binary file we have to interpret the raw bit patterns into the correct type of data for our program. It is perfectly possible to interpret a stream of bytes that were originally written as a character string as a set of floating point numbers. Or course the original meaning will have been lost but the bit patterns could represent either. So when we read binary data it is extremely important that we convert it into the correct data type.

The Struct Module

To encode/decode binary data Python provides a module called struct, short for structure. struct works very much like the format strings we have been using to print mixed data. We provide a string representing the data we are reading and apply it to the byte stream that we are trying to interpret. We can also use struct to convert a set of data to a byte stream for writing, either to a binary file (or even a communications line!).

There are many different conversion format codes but we will only use the integer and string codes here. (You can look up the others on the Python documentation for the struct module.) The codes for integer and string are i, and s respectively. The struct format strings consist of sequences of codes with numbers pre-pended to indicate how many of the items we need. The exception is the s code where the prepended number means the length of the string. For example 4s means a string of four characters (note 4 characters not 4 strings!).

Let's assume we wanted to write the address details, from our Address Book program above, as binary data with the street number as an integer and the rest as a string (This is a bad idea in practice since street "numbers" sometimes include letters!). The format string would look like:

'i34s' # assuming 34 characters in the address!

To cope with multiple address lengths we could write a function to create the binary string like this:

def formatAddress(address): 
    # split breaks a string into a list of 'words'
    fields = address.split()
    number = int(fields[0])
    rest = ' '.join(fields[1:])
    format = "i%ds" % len(rest) #create the format string
    return struct.pack(format, number, rest)

So we used a string method - split() - (more on them in the next topic!) to split the address string into its parts, extract the first one as the number and then use another string method, join to join the remaining fields back together. The length of that string is the number we need in the struct format string. Phew!

formatAddress() will return a sequence of bytes containing the binary representation of our address. Now that we have our binary data let's see how we can write that to a binary file and then read it back again.

Reading & Writing Using Struct

Let's create a binary file containing a single address line using the formatAddress() function defined above. We need to open the file for writing in 'wb' mode, encode the data, write it to the file and then close the file. Let's try it:

import struct
f = file('address.bin','wb')
data = "10 Some St, Anytown, 0171 234 8765"
bindata = formatAddress(data)
print "Binary data before saving: ", repr(bindata)

You can check that the data is indeed in binary format by opening address.bin in notepad. The characters will be readable but the number will not look like 10! In fact it has dissappeared! If you have an editor which can read binary files (e.g vim or emacs) and use that to open address.bin you will see that the start of the file has 4 bytes. The first of these may look like a newline character and the rest are zeros. Now it turns out that, just coincidentally, the numerical value of newline is 10! As we can show using Python:

>>> ord('\n')

The ord function simply returns the ASCII value of a given character. So the first 4 bytes are 10,0,0,0 in decimal (or 0xA,0x0,0x0,0x0 in hexadecimal).

On a 32 bit computer an integer takes up 4 bytes. So the integer value '10' has been converted by the struct module into the 4 byte binary sequence 10,0,0,0. Now on intel micro-processors the byte sequence is to put the least significant byte first so that, reading it in reverse, gives us the true "binary" value: 0,0,0,10.

Which is the integer value 10 expressed as 4 decimal bytes. The rest of the data is basically the original text string and so appears in its normal character format.

Be sure not to save the file from within Notepad since although Notepad can load some binary files it cannot save them as binary, it will try to convert the binary to text and can corrupt the data in the process! It is worth pointing out here that the file extension .bin that we used is purely for our convenience, it has no bearing on whether the file is binary or text format. Some Operating Systems use the extension to determine what programme they will use to open the file, but you can change the extension by simply renaming the file, the content will not change it will still be binary or text whichever it was originally. (You can prove this by renaming a text file in Windows to .exe whereupon Windows will treat the file as an executable, but when you try to run it you will get an error because the text is not really executable binary code! If you now rename it back to .txt the file will open in Notepad exactly as it did before, the content has not been altered at all. - In fact you could even have opened the text in Notepad while it was named as a .exe and it would have worked just as well!)

To read our binary data back again we need to open the file in 'rb' mode, read the data into a sequence of bytes, close the file and finally unpack the data using a struct format string. The question is: how do we tell what the format string looks like? In general we would need to find the binary format from the file definition (there are several web sites which provide this informaton - for example Adobe publish the definition of their common PDF binary format). In our case we know it must be like the one we created in formatAddress(), namely 'iNs' where N is a variable number. How do we determine the value of N?

The struct module provides some helper functions that return the size of each data type, so by firing up the Python prompt and experimenting we can find out how many bytes of data we will get back for each data type:

>>> import struct
>>> print struct.calcsize('i')
>>> print struct.calcsize('s')

Ok, we know that our data will comprise 4 bytes for the number and one byte for each character. So N will be the length of the data minus 4. Let's try using that to read our file:

import struct
f = file('address.bin','rb')
data = f.read()

fmtString = "i%ds" % (len(data) - 4)
number, rest = struct.unpack(fmtString, data)
address = ' '.join((str(number),rest))

print "Address after restoring data:", address

And that's it on binary data files, or at least as much as I'm going to say on the subject. As you can see using binary data introduces several complications and unless you have a very good reason I don't recommend it. But at least if you do need to read a binary file, you can do it (provided you know what the data represented in the first place of course!)

Random Access to Files

The last aspect of file handling that I'll consider is called random access. Random access means moving directly to a particular part of the file without reading all the intervening data. Some programming languages provide a special indexed file type that can do this very quickly but in most languages its built on top of the normal sequential file access that we have been using up till now.

The concept used is that of a cursor that marks the current position within the file, literally how many bytes we are from the beginning. We can move this cursor relative to its current position or relative to the start of the file. We can also ask the file to tell us where the cursor is currently.

By using a fixed linelength (perhaps by padding our data strings with spaces or some other character where necessary) we can jump to the start of a particular line by multiplying the length of a line by the number of lines. This is what gives the impression of random access to the data in the file.

Where am I?

To determine where we are in a file we can use the tell() method of a file. For example if I open a file and read three lines, I can then ask the file how far into the file I am.

Let's look at an example, first I will create a file with 5 lines of text all the same length (the equal length business isn't strictly necessary but it does make life easier!). Then I'll read three lines back and ask where we are. I'll then go back to the beginning, read one line then jump to the third line and print it, jumping over the second line. Like this:

# create 5 lines of 20 chars (+ \n)
testfile = open('testfile.txt','w')
for i in range(5):
    testfile.write(str(i) * 20 + '\n')

# read 3 lines and ask where we are
testfile = open('testfile.txt','r')
for line in range(3):
    print testfile.readline().strip()
position = testfile.tell()
print "At position: ", position, "bytes"

# go back to the beginning
print testfile.readline().strip()
lineLength = testfile.tell()
testfile.seek(2*lineLength)  # go to end of line 2
print testfile.readline().strip()

Note the use of the seek() function to move the cursor. The default operation is to move it to the byte number specified, as shown here, but extra arguments can be provided that change the indexing method used. Also note that the value printed by the first tell() depends on the length of a newline on your platform, on my Windows XP PC it printed 66 indicating that the newline sequence is 2 bytes long (66/3-20). But since this is a platform specific value and I want to make my code portable I've used tell() again, after reading one line, to work out how long each line really is. These kind of "cunning ploys" are often necessary when dealing with platform specific issues!

Some new access modes

Things to remember
  • Open files before using them
  • Files can usually only be read or written but not both at the same time
  • Python's readlines() function reads all the lines in a file, while readline() only reads one line at a time, which may help save memory.
  • However in v2.4 onwards you don't usually need to use either since Python's file/open functions work with for loops.
  • Close files after use.
  • Binary files need the mode flag to end in 'b'
  • tell() and seek() enable pseudo-random access to sequential files
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    If you have any questions or feedback on this page send me mail at: alan.gauld@yahoo.co.uk