Memory Mapping with mmap() Function

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Well, have I got news for ya! Introducing the mmap() function in Python’s standard library the ultimate solution to all your slow-loading file woes.

But before we dive into how this magical function works its magic, let’s first talk about computer memory and why it matters. You see, there are different types of memory that exist in a computer system: primary memory (RAM), secondary memory (hard drives or SSDs), and tertiary memory (external storage devices like USB sticks).

Now, when you open a file using Python’s built-in `open()` function, the contents are loaded into your computer’s RAM. This is great for small files that can fit in memory, but what about those massive ones? Well, that’s where mmap comes in! With this function, we can map a portion of a file directly to our system’s physical memory, allowing us to access it much faster than if we were reading from disk.

So how do you use the mmap() function? It’s actually pretty simple: just replace your `open()` call with this bad boy! Here’s an example:

# Import the mmap module
import mmap

# Define the filename of the file we want to map
filename = 'my_big_file.txt'

# Open the file in read-only mode using the `with` statement
with open(filename, 'rb') as f:
    # Get the size of the file in bytes using the `os` module
    filesize = os.path.getsize(filename)
    
    # Create a memory-mapped object with read/write access to the entire file
    # `mmap_obj` will be used to access the file in memory
    # `f.fileno()` returns the file descriptor of the opened file
    # `0` indicates that we want to map the entire file
    # `access=mmap.ACCESS_READWRITE` specifies that we want both read and write access to the file
    mmap_obj = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READWRITE)

That’s it! Now you can use `mmap_obj` just like a regular string or bytearray object but with the added benefit of lightning-fast I/O speeds. And if you only need to modify a portion of the file, you can create a memory-mapped object for that specific section using the same function:

# Open the file and get its size in bytes
with open('my_big_file.txt', 'rb') as f:
    filesize = os.path.getsize(filename) # Get the size of the file in bytes
    
    # Calculate the starting byte for the section we want to modify (in this case, 10-15)
    start_byte = 10 * 8   # Each character is 8 bytes in Python 3
    
    # Create a memory-mapped object with read/write access to that specific section
    mmap_obj = mmap.mmap(f.fileno(), filesize, access=mmap.ACCESS_READWRITE) # Create a memory-mapped object with read/write access to the file
    start_section = mmap_obj[start_byte:start_byte + 6]   # Get the current contents of our section
    
    # Modify the section in memory (in this case, changing "hello" to "python")
    for i in range(len(start_section)): # Loop through the section
        if start_section[i] == b'l': # Check if the current character is 'l'
            mmap_obj[start_byte + i] = b'p'   # Change the 'l' character at index 10 to a 'p'
    
    # Save our changes back to disk (this is optional you can also modify the section in memory and then close the file without saving)
    mmap_obj.flush() # Save the changes made to the memory-mapped object back to the file

And that’s it! With these simple examples, you should now be able to use Python’s mmap function like a pro. So go ahead and say goodbye to those slow-loading files because with memory mapping, your computer will thank you for it!

But remember, as always: don’t forget to close the file when you’re done using it (or else you might run into some unexpected issues). And if you have any questions or comments, feel free to reach out to me on Twitter @Python_Sarcastic.

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