Python在for循环的每次迭代中为范围函数生成随机步骤

Question

I am trying to split a string into small chunks of random sizes. For example,

string = 'qwertyuiopp' into ['qw','e','rty,'u','iopp']

for record in SeqIO.parse(args.fasta_file , "fasta"):
    step = 200
    for i in range(0, len(record),step):
        oline = ">"+record.id+"_"+str(i+1)+"\n"
        ofname.write(oline)
        step = random.randrange(200,2000)
        if len(record.seq[i:i+step]) >= 200:
            oline= str(record.seq[i:i+step])+"\n"
            ofname.write(oline)
        i=i+step
        else:
            oline= str(record.seq[i-200:])+"\n"
            ofname.write(oline)
        i=len(record)+1

I figure the problem is that the step in range function remains constant(200) inspite of modifying the value of step using randrange. but I dont know how I should go about this. Thank for any help.

Answer 1

range function creates a list as soon as the for loop is encountered. It does not get evaluated for every iteration. You can use a while loop for this purpose.

import random
step = 1
count = 0
while count < len(record):     
    print count
    step = random.randint(1, 5)
    count += step

Answer 2

The range function does not remember how it got the values it was constructed from. When you call range(0, len(record), step) that creates either the range object range(0, 1000, 200) (Python 3.x) or the list [0, 200, 400, 600, 800] (Python 2.x). Either way, changing step later isn't going to change anything.

You could build a custom iterator that that lets you reset its step, or one that changes its step and iterates (start, end) or (start, step) pairs. But that may be a little advanced for you.*

In that case, when you can't find or write an iterator to loop over, you have to go to a while loop, and update your loop variable manually.

It's worth noting that you're already trying to update your loop variable manually, with that i = i + step . That's usually a bad idea inside a for loop—but inside a while loop, it's exactly what you want. So really, just change this line:

for i in range(0, len(record),step):

… to these two lines:

i = 0
while i < len(record):

There are other problems in your code that you have to fix (including at least two IndentationError s), but you can fix them once you get past this.

One last thing: Whether you've got a for loop or a while loop, if you want to break out early, the way to do it is a break statement, not by trying to change the loop conditions so the next test will fail. So, replace that i=len(record)+1 at the end with break as well.

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* Also, the obvious implementation for such an iterator would be the same kind of while loop anyway…