diff --git a/scripts/extract_tracts.py b/scripts/extract_tracts.py
index d7327e6..40e479e 100755
--- a/scripts/extract_tracts.py
+++ b/scripts/extract_tracts.py
@@ -23,6 +23,9 @@
 import logging
 import re
 import os
+import shutil
+
+from io import StringIO
 
 logging.basicConfig(format="%(levelname)s (%(name)s %(lineno)s): %(message)s")
 logger = logging.getLogger(__name__)
@@ -155,10 +158,14 @@ def extract_tracts(vcf=str, msp=str, num_ancs=int, output_dir=None, output_vcf=N
                 pop_genos = {}
 
                 for i in range(num_ancs):  # Write entries into each output files' list
-                    output_lines[f"dos{i}"] = dos_anc_out
-                    output_lines[f"ancdos{i}"] = dos_anc_out
+                    output_lines[f"dos{i}"] = StringIO()
+                    output_lines[f"dos{i}"].write(dos_anc_out)
+                    output_lines[f"ancdos{i}"] = StringIO()
+                    output_lines[f"ancdos{i}"].write(dos_anc_out)
+
                     if output_vcf:
-                        output_lines[f"vcf{i}"] = vcf_out
+                        output_lines[f"vcf{i}"] = StringIO()
+                        output_lines[f"vcf{i}"].write(vcf_out)
 
                 # optimized for quicker runtime - only move to next line when out of the current msp window
                 # saves the current line until out of the window, then checks next line. VCF and window switches file should be in incremental order.
@@ -174,7 +181,7 @@ def extract_tracts(vcf=str, msp=str, num_ancs=int, output_dir=None, output_vcf=N
                         break  # when get to the end of the msp file, stop
                     # chm, spos, epos, sgpos, egpos, nsnps, calls
                     ancs_entry = ancs.strip().split("\t", 6)
-                    calls = ancs_entry[6].split("\t")
+                    calls = [int(x) for x in ancs_entry[6].split("\t")]
                     window = (ancs_entry[0], int(ancs_entry[1]), int(ancs_entry[2]))
                     if row[0] == window[0] and window[1]> pos:
                         skip_line=True #Skip VCF line
@@ -187,15 +194,15 @@ def extract_tracts(vcf=str, msp=str, num_ancs=int, output_dir=None, output_vcf=N
                 for i, geno in enumerate(genos):
                     geno_parts = geno.split(":")[0].split("|")  # assert incase eagle leaves some genos unphased
                     geno_a,geno_b = map(str, geno_parts[:2])
-                    call_a = str(calls[2*i])
-                    call_b = str(calls[2*i + 1])
+                    call_a = calls[2*i]
+                    call_b = calls[2*i + 1]
 
                     counts     = {anc: 0 for anc in range(num_ancs)}
                     anc_counts = {anc: 0 for anc in range(num_ancs)}
                     for j in range(num_ancs):
                         if output_vcf:
                             pop_genos[j] = ""
-                        if call_a == str(j):
+                        if call_a == j:
                             if output_vcf:
                                 pop_genos[j] += geno_a
                             anc_counts[j] += 1
@@ -205,7 +212,7 @@ def extract_tracts(vcf=str, msp=str, num_ancs=int, output_dir=None, output_vcf=N
                             if output_vcf:
                                 pop_genos[j] += "."
 
-                        if call_b == str(j):
+                        if call_b == j:
                             if output_vcf:
                                 pop_genos[j] += "|" + geno_b
                             anc_counts[j] += 1
@@ -215,19 +222,25 @@ def extract_tracts(vcf=str, msp=str, num_ancs=int, output_dir=None, output_vcf=N
                             if output_vcf:
                                 pop_genos[j] += "|."
 
-                        output_lines[f"dos{j}"] += "\t" + str(counts[j])
-                        output_lines[f"ancdos{j}"] += "\t" + str(anc_counts[j])
+                        output_lines[f"dos{j}"].write("\t" + str(counts[j]))
+                        output_lines[f"ancdos{j}"].write("\t" + str(anc_counts[j]))
                         if output_vcf:
-                            output_lines[f"vcf{j}"] += "\t" + pop_genos[j]
+                            output_lines[f"vcf{j}"].write("\t" + pop_genos[j])
 
                 for j in range(num_ancs):
-                    output_lines[f"dos{j}"] += "\n"
-                    output_lines[f"ancdos{j}"] += "\n"
-                    files[f"dos{j}"].write(output_lines[f"dos{j}"])
-                    files[f"ancdos{j}"].write(output_lines[f"ancdos{j}"])
+                    output_lines[f"dos{j}"].write("\n")
+                    output_lines[f"ancdos{j}"].write("\n")
+
+                    output_lines[f"dos{j}"].seek(0)
+                    output_lines[f"ancdos{j}"].seek(0)
+                    shutil.copyfileobj(output_lines[f"dos{j}"], files[f"dos{j}"])
+                    shutil.copyfileobj(output_lines[f"ancdos{j}"], files[f"ancdos{j}"])
+                    
+
                     if output_vcf:
-                        output_lines[f"vcf{j}"] += "\n"
-                        files[f"vcf{j}"].write(output_lines[f"vcf{j}"])
+                        output_lines[f"vcf{j}"].write("\n")
+                        output_lines[f"vcf{j}"].seek(0)
+                        shutil.copyfileobj(output_lines[f"vcf{j}"], files[f"vcf{j}"])
 
     logger.info("Finished extracting tracts per %d ancestries", num_ancs)