Python int to one bytes

Convert a Python int into a big-endian string of bytes

I have a non-negative int and I would like to efficiently convert it to a big-endian string containing the same data. For example, the int 1245427 (which is 0x1300F3) should result in a string of length 3 containing three characters whose byte values are 0x13, 0x00, and 0xf3. My ints are on the scale of 35 (base-10) digits. How do I do this?

9 Answers 9

If you don’t want to specify the size

>>> n = 1245427 >>> n.to_bytes((n.bit_length() + 7) // 8, 'big') or b'\0' b'\x13\x00\xf3' 

If you don’t mind specifying the size

>>> (1245427).to_bytes(3, byteorder='big') b'\x13\x00\xf3' 

Am I right in thinking the (length + 7) // 8 part is the same as doing math.ceil(length / 8) ? If so, I think it’d be clearer to use that option

@JanusTroelsen They are the same, as long as the n.bit_length() is a reasonable size (you need around a quadrillion bits for there to be an issue, which means that n is going to be unbelievably large). In my situation readability is much more important than a performance increase, and numbers will never be that big.

@Kebman why would they be valid UTF-8? I don’t see why that would be a case. Maybe ask a new well-written question and ping me, and I’ll try to respond.

import struct print(struct.pack('>I', your_int)) 

‘>I’ is a format string. > means big endian and I means unsigned int. Check the documentation for more format chars.

struct.pack returns a fixed length string and doesn’t seem to have facilities for handling large ints. I suppose I could break my int into powers of 2^32, run it through struct.pack(), and reassemble the result, but that seems like a lot of work. do you know of a simpler way?

I couldn’t find a library to handle arbitrary long ints. I think you will have to implement it yourself. Other answers contain implementations.

Ayman, note that Python has built-in support for arbitrarily long ints, so you don’t need a library. In Python 3, there will only be the int type, but even now in Python 2.4+, int s are automatically converted to Python long s when they overflow 32 bits (signed).

benhoyt, thanks for the comment. I’m aware of this. I was talking about handling the conversion of arbitrary long ints to big endian. Not handling them in general.

This is fast and works for small and (arbitrary) large ints:

def Dump(n): s = '%x' % n if len(s) & 1: s = '0' + s return s.decode('hex') print repr(Dump(1245427)) #: '\x13\x00\xf3' 

As a variant of the above, one can replace if len(s) & 1 with if len(s) % 2 (both are ture if there are an odd amount of hexadecimal characters), and ‘%x’ % n with ‘<0:x>‘.format(n) (both of which format the number as a hexadecimal string).

Probably the best way is via the built-in struct module:

>>> import struct >>> x = 1245427 >>> struct.pack('>BH', x >> 16, x & 0xFFFF) '\x13\x00\xf3' >>> struct.pack('>L', x)[1:] # could do it this way too '\x13\x00\xf3' 

Alternatively — and I wouldn’t usually recommend this, because it’s mistake-prone — you can do it «manually» by shifting and the chr() function:

>>> x = 1245427 >>> chr((x >> 16) & 0xFF) + chr((x >> 8) & 0xFF) + chr(x & 0xFF) '\x13\x00\xf3' 

Out of curiosity, why do you only want three bytes? Usually you’d pack such an integer into a full 32 bits (a C unsigned long ), and use struct.pack(‘>L’, 1245427) but skip the [1:] step?

Источник

Convert int to bytes in Python

In this tutorial, we will look at how to convert an int type object to a bytes type object in Python with the help of some examples.

How to convert int to bytes in Python?

You can use the int class method int.to_bytes() to convert an int object to an array of bytes representing that integer. The following is the syntax –

# int to bytes int.to_bytes(length, byteorder, signed)

It takes the following arguments –

• length – The number of bytes to use to represent the integer. If the integer is not representable with the given number of bytes, an OverflowError is raised.
• byteorder – Determines the byte order used to represent the integer. Use ‘big’ as the byte order to have the most significant byte at the beginning of the byte array. Use ‘little’ as the byte order to have the most significant byte at the end of the byte array.
• signed – Determines wheter to use two’s compliment to represent the integer. It is an optional parameter and is False by default. Helpful in converting signed integers to bytes.

Examples

Let’s look at some examples of using the int.to_bytes() function to convert an integer to bytes.

Using ‘big’ as the byteorder

Let’s convert the integer value 7 to a byte array of length 2 and with “big” as the byteorder.

# integer variable num = 7 # integer to bytes num_bytes = num.to_bytes(2, byteorder='big') # display result and type print(num_bytes) print(type(num_bytes))

We get the returned value as bytes with the most significant byte at the beginning.

Using ‘little’ as the byteorder

Let’s use the same example as above but with “little” as the byteorder

# integer variable num = 7 # integer to bytes num_bytes = num.to_bytes(2, byteorder='little') # display result and type print(num_bytes) print(type(num_bytes))

We get the most significant byte at the end of the byte array.

Negative Integers to bytes with signed=True

If you use the default signed=False on a negative integer, you will get an OverflowError .

# integer variable num = -7 # integer to bytes num_bytes = num.to_bytes(2, byteorder='big') # display result and type print(num_bytes) print(type(num_bytes))

--------------------------------------------------------------------------- OverflowError Traceback (most recent call last) Input In [11], in 2 num = -7 3 # integer to bytes ----> 4 num_bytes = num.to_bytes(2, byteorder='big') 5 # display result and type 6 print(num_bytes) OverflowError: can't convert negative int to unsigned

To convert negative integers to bytes with the int.to_bytes() function, pass signed=True . It will use two’s complement to represent the integer.

# integer variable num = -7 # integer to bytes num_bytes = num.to_bytes(2, byteorder='big', signed=True) # display result and type print(num_bytes) print(type(num_bytes))

We get the bytes for the negative integer.

For more on the int.to_bytes() function, refer to its documentation.

You might also be interested in –

Subscribe to our newsletter for more informative guides and tutorials.
We do not spam and you can opt out any time.

Author

Piyush is a data professional passionate about using data to understand things better and make informed decisions. He has experience working as a Data Scientist in the consulting domain and holds an engineering degree from IIT Roorkee. His hobbies include watching cricket, reading, and working on side projects. View all posts

Источник

Python int to one bytes

Last updated: Jun 23, 2023
Reading time · 3 min

# Table of Contents

# How to convert Int to Bytes in Python

Use the int.to_bytes() method to convert an integer to bytes in Python.

The method returns an array of bytes representing an integer.

Copied! 
num = 2048 my_bytes = num.to_bytes(2, byteorder='big') print(my_bytes) # 👉️ b'\x08\x00' 

If your integer is not stored in a variable, make sure to wrap it in parentheses before calling to_bytes() .

Copied! 
my_bytes = (2048).to_bytes(2, byteorder='big') print(my_bytes) # 👉️ b'\x08\x00' 

The int.to_bytes method returns an array of bytes representing an integer.

The integer is represented using length bytes and defaults to 1 .

An OverflowError is raised if the integer cannot be represented using the given number of bytes.

Copied! 
my_bytes = (1024).to_bytes(2, byteorder='big') print(my_bytes) # 👉️ b'\x04\x00' my_bytes = (1024).to_bytes(10, byteorder='big') print(my_bytes) # 👉️ b'\x00\x00\x00\x00\x00\x00\x00\x00\x04\x00' 

The second argument we passed to the to_bytes() method determines the byte order that is used to represent the integer.

The byteorder argument defaults to big .

If the byteorder argument is set to big , then the most significant byte is at the beginning of the byte array.

If you set the byteorder argument to little , the most significant byte is at the end of the byte array.

Copied! 
my_bytes = (1024).to_bytes(2, byteorder='little') print(my_bytes) # 👉️ b'\x00\x04' my_bytes = (1024).to_bytes(10, byteorder='little') print(my_bytes) # 👉️ b'\x00\x04\x00\x00\x00\x00\x00\x00\x00\x00' 

# Creating a reusable function to convert an integer to bytes and vice versa

You can also create a reusable function that converts integers to bytes.

Copied! 
def int_to_bytes(integer): return integer.to_bytes((integer.bit_length() + 7) // 8, 'big') print(int_to_bytes(65)) # 👉️ b'A' print(int_to_bytes(1024)) # 👉️ b'\x04\x00' print(int_to_bytes(2048)) # 👉️ b'\x08\x00' 

The int_to_bytes function takes an integer as a parameter and converts it to a bytes object.

Conversely, if you need to convert a bytes object to an integer, use the int.from_bytes() method instead.

Copied! 
def int_from_bytes(bytes_obj): return int.from_bytes(bytes_obj, byteorder='big') print(int_from_bytes(b'A')) # 👉️ b'A' print(int_from_bytes(b'\x04\x00')) # 👉️ b'\x04\x00' print(int_from_bytes(b'\x08\x00')) # 👉️ b'\x08\x00' 

The int.from_bytes() method returns the integer represented by the given byte array.

The first argument the method takes must be a bytes-like object or an iterable that produces bytes.

The byteorder argument determines which byte order is used to represent the integer.

The default value is big which means that the most significant byte is at the beginning of the byte array.

If the byteorder argument is set to «little» , then the most significant byte is at the end of the byte array.

# Converting signed (negative) integers to bytes in Python

The examples above only work for unsigned (non-negative integers).

If you need to convert signed integers to bytes, use the following function instead.

Copied! 
def int_to_bytes(integer): return integer.to_bytes( length=(8 + (integer + (integer  0)).bit_length()) // 8, byteorder='big', signed=True ) print(int_to_bytes(-1024)) # 👉️ b'\xfc\x00' print(int_to_bytes(-2048)) # 👉️ b'\xf8\x00' 

Calculating the length argument when converting signed (negative) integers to bytes is a bit more complicated.

The signed argument determines whether two’s complement is used to represent the integer.

If signed is False and a negative integer is supplied, an OverflowError is raised.

By default, the signed argument is set to False .

The following function can be used if you need to convert signed bytes to integers.

Copied! 
def int_from_bytes(binary_data): return int.from_bytes(binary_data, byteorder='big', signed=True) print(int_from_bytes(b'\xfc\x00')) # -1024 print(int_from_bytes(b'\xf8\x00')) # -2048 

The signed argument indicates whether two’s complement is used to represent the integer.

# Converting the Integer to a String and then Bytes

If you need to convert the integer to a string and then bytes, use the str.encode method.

Copied! 
num = 2048 my_bytes = str(num).encode(encoding='utf-8') print(my_bytes) # 👉️ b'2048' 

We passed the integer to the str class to convert it to a string and then used the str.encode() method to convert the string to bytes.

The same can be achieved by using the bytes class.

Copied! 
num = 2048 my_bytes = bytes(str(num), encoding='utf-8') print(my_bytes) # 👉️ b'2048' 

If you need to convert the bytes object back to an integer, use the bytes.decode method and the int() class.

Copied! 
num = 2048 my_bytes = str(num).encode(encoding='utf-8') print(my_bytes) # 👉️ b'2048' my_int = int(my_bytes.decode(encoding='utf-8')) print(my_int) # 👉️ 2048 

The bytes.decode() method converts the bytes object to a string.

The last step is to use the int() class to convert the string to an integer.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

I wrote a book in which I share everything I know about how to become a better, more efficient programmer.

Источник