Python merges concrete instances of files

Two USES are supported:
(1) merge all files under a certain folder (ignore non-file entries such as folders)
(2) merge multiple files as shown.

import sys
import os
'''
    usage(1): merge_files pathname
              pathname is directory and merge files in pathname directory
    usage(2): merge_files file1 file2 [file3[...]]
'''
FILE_SLIM = (256*(1024*1024)) #256M match 2**n
def merge_files(fileslist,mfname):
    global FILE_SLIM
    p_fp = open(mfname,"wba")
    for file in fileslist:
        with open(file,"rb") as c_fp:
            fsize = os.stat(file).st_size
            count = fsize&FILE_SLIM
            while count>0:
                p_fp.write(c_fp.read(FILE_SLIM))
                fsize -= FILE_SLIM
                count -= 1
            p_fp.write(c_fp.read())
    p_fp.close
def main():
    argc = len(sys.argv) - 1
    fileslist = []
    if argc == 2:
        dir_name = os.path.realpath(sys.argv[1])
        assert(os.path.isdir(dir_name))
        file_dir = os.listdir(dir_name)
        fileslist = [os.path.join(dir_name,file) for file in file_dir if os.path.isfile(os.path.join(dir_name,file))]
        print(fileslist)
    elif argc >=3:
        fileslist = [os.path.realpath(sys.argv[index]) for index in range(1,argc) if os.path.isfile(os.path.realpath(sys.argv[index]))]
    merge_files(fileslist,sys.argv[argc])
if __name__ == '__main__':
    main()