Why is this Python script failing, when it cat cat the file

Question

It does the 'cat' cmd but then can't find the same file, which DOES exist...

MBAir3:batch Fred$ cat c.py

    import os
    host_name = os.system('hostname > ~/batch/fjhostname.txt')
    print('cat of ~/batch/fjhostname.txt')
    junk=os.system('cat ~/batch/fjhostname.txt')
    with open("~/batch/fjhostname.txt", "r") as h:
      for line in h:
        fjhostname=line
    junk = os.system('rm ~/batch/fjhostname.txt') 
    

Result:

    MBAir3:batch Fred$ python3 c.py
    cat of ~/batch/fjhostname.txt
    MBAir3
    Traceback (most recent call last):
      File "c.py", line 5, in <module>
        with open("~/batch/fjhostname.txt", "r") as h:
    FileNotFoundError: [Errno 2] No such file or directory: '~/batch/fjhostname.txt'
    
    Files after run: (fjhostname.txt does exist in ~/batch)
    
        MBAir3:batch Fred$ ls
        B4 copy.py  B4.py.FJSNAP1   B4doc06     aaa     fjhostname.txt
        B4.py       B4.py.FJSNAP2   B4doc89     c.py        git