[ Python 常見問題 ] Random string generation with upper case letters and digits in Python

Source From Here 
Question 
I want to generate a string of size N. It should be made up of numbers and uppercase English letters such as: 

* 6U1S75
* 4Z4UKK
* U911K4

How-To 
Answer in one line: 

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1. ''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(N))  

A more secure version; see http://stackoverflow.com/a/23728630/2213647: 

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1. ''.join(random.SystemRandom().choice(string.ascii_uppercase + string.digits) for _ in range(N))  

In details, with a clean function for further reuse: 

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1. >>> import string  
2. >>> import random  
3. >>> def id_generator(size=6, chars=string.ascii_uppercase + string.digits):  
4. ...    return ''.join(random.choice(chars) for _ in range(size))  
5. ...  
6. >>> id_generator()  
7. 'G5G74W'  
8. >>> id_generator(3, "6793YUIO")  
9. 'Y3U'  

How does it work ? 
We import string, a module that contains sequences of common ASCII characters, and random, a module that deals with random generation. 

string.ascii_uppercase + string.digits just concatenates the list of characters representing uppercase ASCII chars and digits: 

>>> string.ascii_uppercase
'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
>>> string.digits
'0123456789'
>>> string.ascii_uppercase + string.digits
'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'

Then we use a list comprehension to create a list of 'n' elements: 

>>> range(4) # range create a list of 'n' numbers
[0, 1, 2, 3]
>>> ['elem' for _ in range(4)] # we use range to create 4 times 'elem'
['elem', 'elem', 'elem', 'elem']

Instead of asking to create 'n' times the string elem, we will ask Python to create 'n' times a random character, picked from a sequence of characters: 

>>> random.choice("abcde")
'a'
>>> random.choice("abcde")
'd'
>>> random.choice("abcde")
'b'

Therefore random.choice(chars) for _ in range(size) really is creating a sequence of size characters. Characters that are randomly picked from chars: 

>>> [random.choice('abcde') for _ in range(3)]
['a', 'b', 'b']

Then we just join them with an empty string so the sequence becomes a string: 

>>> ''.join(['a', 'b', 'b'])
'abb'
>>> [random.choice('abcde') for _ in range(3)]
['d', 'c', 'b']
>>> ''.join(random.choice('abcde') for _ in range(3))
'dac'

Supplement 
* [ Python 文章收集 ] Iterators & Generators