1. What is hashable (hashable)
In a nutshell , Hashable data types , That is, immutable data structure ( character string str、 Tuples tuple、 Object set objects).
2. What is the function of hash
It is a process of transforming large data into small data , It can even be just a number , So that we can query it in a fixed time complexity , therefore , Hashes are important for efficient algorithms and data structures .
3. What is not hashable (unhashable)
Empathy , Non hashable data type , Changeable data structure ( Dictionaries dict, list list, aggregate set).
4. hash(object)
hash() Used to get an object （ String or number, etc ） Hash value of . Returns the hash value of the object .
1. brief introduction
Hash （hash） Also translated as hash .Hash Algorithm , Is to input an indefinite length , It is transformed into a fixed length output by hash function , That is, the hash value .
This hash transformation is a one-way operation , It is irreversible, that is, it cannot restore the input information according to the hash value , So strictly speaking Hash The algorithm is a message digest algorithm , It's not an encryption algorithm . common hash The algorithms are ：SM3、MD5、SHA-1 etc. .
2. application
Hash It is mainly used in the field of data structure and cryptography .
In different application scenarios ,hash The choice of function will also be emphasized . For example, when managing data structures , The main consideration is the rapidity of operation , And make sure that hash Uniform distribution ; In cryptography, we should give priority to anti-collision , Avoid two different plaintext hash The same value occurs .
2.1 Applications in the field of cryptography
In cryptography ,Hash The algorithm is mainly used for message digest and signature , let me put it another way , It is mainly used to check the integrity of the whole message . For example, some login websites do not directly store user passwords in clear text , What's stored is the process of hash Summary of passwords processed （hash value ）, When users log in, they only need to compare whether the summary of input plaintext is the same as that stored in the database ; Even if hackers invade or maintainers access the database, they cannot obtain the user's password plaintext , Greatly improved security .
2.2 Application in data structure
Use Hash The data structure of the algorithm is called hash table , Also called a hash table , Mainly to improve the efficiency of query . It accesses records by mapping key values to a location in a table , To speed up the search . This mapping function is hash function , The array of records is called a hash table . When applied in data structures , Sometimes it needs higher computing speed and weakens the consideration of collision resistance , You can use your own hash function .
3. hash()
① Same value , Different types , Same hash value
>>> n = hash(1)
>>> n
1
>>> n = hash(1.0)
>>> n
1
② The hash value of the same string at the same run time is the same , But the hash value of different runs is different
Python 3.7.2 (tags/v3.7.2:9a3ffc0492, Dec 23 2018, 23:09:28) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> m = hash('Vivian')
>>> m
-2338955002766744599 # First hash value
>>> exit()
C:\Users\Administrator>python
Python 3.7.2 (tags/v3.7.2:9a3ffc0492, Dec 23 2018, 23:09:28) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> m = hash('Vivian')
>>> m
2113605006884176523 # Second hash value
This is because Python String hash The algorithm has a random generation at startup secret prefix/suffix The mechanism of , There is randomization ： Enter... For the same string , Different interpreter processes get hash The result may be different . Therefore, when it is necessary to do reproducible and consistent across processes hash, Need to use hashlib modular .
hashlib Provides common summary algorithms , Such as MD5,SHA1 wait
notes ：coding：utf-8
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import hashlib
# md5
m = hashlib.md5()
m.update(b'Vivian')
print(m.digest()) # Returns the binary hash value b'\xe5if5\x05\x02\x1a!wR\xd6\xfa(\xae\r\xcb'
print(m.hexdigest()) # Returns the hexadecimal hash value e569663505021a217752d6fa28ae0dcb
m.update(' Hello '.encode('utf-8'))
print(m.hexdigest()) # 6ea8d7a29ea0706bdaca285e1d2ddd17
# m The value after two encryptions be equal to The value encrypted once after two string splicing
n = hashlib.md5()
n.update('Vivian Hello '.encode('utf-8'))
print(n.hexdigest()) # 6ea8d7a29ea0706bdaca285e1d2ddd17
# sha1
m = hashlib.sha1()
m.update(b'Vivian')
print(m.hexdigest()) # 587bf2d7314da0ae33623bed16d872620b2768be
# sha256
m = hashlib.sha256()
m.update(b'Vivian')
print(m.hexdigest()) # 2f7caf725fb93a456e84100bdf98b91405e62f6e7ca99bc4e0b4a1993bf9e5c0
# sha512
m = hashlib.sha512()
m.update(b'Vivian')
print(m.hexdigest())
# d75e73edd6ab1801e3dbfcd5c504dca16f3c252f750411da7fdddc9b60013ad97844b6f86de29d3f9a6d07d2c93a53d6c9de9b548b8697f3c2494857176011dc
