if you mean extracting the reads aligned to chr1 you can use

import pysam
samfile = pysam.AlignmentFile("your.bam", "rb")
for read in samfile.fetch('chr1', chr_beginning, chr_end):
          print read
samfile.close()

and if you want extract your chromosome in bam format using samtools

samtools index your.bam
samtools view -bh your.bam chr1  > small.bam