尝试从文件中读取并写入另一个文件。生成的文件为空

我有一个.fastq文件，我试图从中读取并生成其反向补码。运行我的脚本后，生成的文件为空。

这是Python 3.7。我正在使用函数rev_comp来完成此任务。这是我已编写的代码。

import gzip
import re

Reverse Complement Function

def rev_comp(dna):

dna_upper = dna.upper() #Ensures sequence is upper-case
dna_rev = dna[::-1] #Reverses the string

conversion = {'A':'T','C':'G','G':'C','T':'A','Y':'R','R':'Y',\
       'S':'S','W':'W','K':'M','M':'K','B':'V','V':'B',\
       'D':'H','H':'D','N':'N','-':'-'}

revcomp = ''

for i in dna_rev:
    revcomp += conversion[i]
    reverse = open("Script_result.txt", 'w')
    reverse.write(revcomp)

reverse.close()

"""

String finding function

def string_find(string):"""

x = input("Enter filename (with extension) of the DNA sequence: ")

if x.endswith(".gz"): #Condition for gzip files

with gzip.open(x, 'rb') as f:
    file_content = f.read()
    new_file = open("unzipped.fasta", 'w')
    new_file.write(str(file_content))
    print("unzipped.fasta written to directory")
    rev_comp(new_file)

with open(x, 'r') as xfile:

xread = xfile.readlines()
seq_string = ''
if x.endswith(".fasta"):   #condition for fasta files
    for i in xread:
        if not i.startswith('>'):
            seq_string = seq_string + i.strip('\n')

if x.endswith(".fastq"):        #condition for fastQ files
    for i in range(1,len(xread),4):
        seq_string = seq_string + xread[i].strip('\n')

rev_comp(seq_string)
我期待生成一个文本文件与fastq文件中的序列反向补充。但是，结果 Script_result.txt是空白的