[Python] how to randomly add a list of sequences into a text body

This is one of my first tasks with actual Python code.

What I need to do is to import a set of (FASTA) sequences, select those with a length between 400 and 500 (base pairs) characters, and randomly pick 100 of those to be added into another (FASTA genome) text body — again at random.

One thing to consider is that I should not have any of those 100 sequences be added within another, so that they can eventually be queried for. In fact, it would be ideal to know also the position where each sequence has been added.

This is my go at the problem, again apologies for the limited coding skills, but I'm no expert with the language. Any help is much appreciated! I'm mostly struggling with the last part, as I managed to add a sequence in another, but I cannot do so for a list object, see below for code and error.

###library import
from Bio import SeqIO
import inspect
import random

###sequences handling
input_file = open("hs_sequences.txt")
my_dict = SeqIO.to_dict(SeqIO.parse(input_file, "fasta"))
#my_dict

###compute sequence length
l = []
for i in my_dict.values():
l.append(len(i))
#l

###select sequences based on range-length estimates of abundance
seq_of_choice = [seq for seq in s if 400 < len(seq) < 500]

###import FASTA
def fasta_reader(filename):
from Bio.SeqIO.FastaIO import FastaIterator
with open(filename) as handle:
for record in FastaIterator(handle):
yield record

def custom_print(string):
counter=0
res=""
for char in string:
if counter==60:
print(res)
counter=0
res=""
continue
res+=char
counter+=1

for entry in fasta_reader("hg37_chr1-1000l.fna"):
print(str(entry.id))
custom_print(str(entry.seq))

body = str(entry.seq)

###import full genome
#example_full = SeqIO.index("hg37_23-only.fna", "fasta")
#example_full

###randomly selects 100 sequences and adds them to the FASTA
def insert (source_str, insert_str, pos):
return source_str[os] + insert_str + source_str[pos:]

i = 1
hundred_seqs = []
while i <= 100:
hundred_seqs.append(random.choice(seq_of_choice))
i += 1
#hundred_seqs

custom_print(insert(body, hundred_seqs, random.randint(0, len(body))))


In this case the error is

TypeError: can only concatenate str (not "list") to str
​

but, of course, it works if I use hundred_seqs[1]

I can provide a link to the files if necessary, the .txt is quite small but the FASTA genome isn't...

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