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    Python struct pack, unpack

    Python struct module is capable of performing the conversions between the Python values and C structs, which are represented as Python Strings.

    Python struct

    • Python struct module can be used in handling binary data stored in files, database or from network connections etc.
    • It uses format Strings as compact descriptions of the layout of the C structs and the intended conversion to/from Python values.

    Functions

    There are five important functions in struct module – pack(), unpack(), calcsize(), pack_into() and unpack_from(). In all these functions, we have to provide the format of the data to be converted into binary. Some of the popular format characters are:

    • ?: boolean
    • h: short
    • l: long
    • i: int
    • f: float
    • q: long long int

    You can get the complete list of format characters here. Let’s start looking into struct module functions one by one.

    Python struct.pack()

    This function packs a list of values into a String representation of the specified type. The arguments must match the values required by the format exactly. Let’s quickly look at struct pack() example:

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    import struct

var = struct.pack('hhl', 5, 10, 15)
print(var)
 
var = struct.pack('iii', 10, 20, 30)
print(var)

    Python struct.unpack()

    This function unpacks the packed value into its original representation with the specified format. This function always returns a tuple, even if there is only one element. Let’s quickly look at struct unpack() function example:

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    import struct
var = struct.pack('hhl', 5, 10, 15)
print(var)
print(struct.unpack('hhl', var))

    Python struct calcsize()

    This function calculates and returns the size of the String representation of struct with a given format. Size is calculated in terms of bytes. Let’s quickly look at an example code snippet:

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    import struct
 
var = struct.pack('hhl', 5, 10, 15)
print(var)
print("Size of String representation is {}.".format(struct.calcsize('hhl')))

    Python struct pack_into(), unpack_from()

    These functions allow us to pack the values into string buffer and unpack from a string buffer. These functions are introduced in version 2.5.

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    import struct
# ctypes is imported to create a string buffer
import ctypes

# As shown in previous example
size = struct.calcsize('hhl')
print(size)

# Buffer 'buff' is created from ctypes
buff = ctypes.create_string_buffer(siz)

# struct.pack_into() packs data into buff and it doesn't return any value
# struct.unpack_from() unpacks data from buff, returns a tuple of values
print(struct.pack_into('hhl', buff, 0, 5, 10, 15))
print(struct.unpack_from('hhl', buff, 0))

    Conclusion

    That’s all for a short introduction of python struct module.

    Source: digitalocean.com

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