Interacting with Hadoop HDFS using Python codes

This post will go through the following:

1. Introducing python “subprocess” module
2. Running HDFS commands with Python
3. Examples of HDFS commands from Python

1-Introducing python “subprocess” module

The Python “subprocess” module allows us to:

• spawn new Unix processes
• connect to their input/output/error pipes
• obtain their return codes

To run UNIX commands we need to create a subprocess that runs the command. The recommended approach to invoking subprocesses is to use the convenience functions for all use cases they can handle. Or we can use the underlying Popen interface can be used directly.

2-Running HDFS commands with Python

We will create a Python function called run_cmd that will effectively allow us to run any unix or linux commands or in our case hdfs dfs commands as linux pipe capturing stdout and stderr and piping the input as list of arguments of the elements of the native unix or HDFS command. It is passed as a Python list rather than a string of characters as you don't have to parse or escape characters.

# import the python subprocess module
import subprocess


def run_cmd(args_list):
"""
run linux commands
"""
# import subprocess
print('Running system command: {0}'.format(' '.join(args_list)))
proc = subprocess.Popen(args_list, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
s_output, s_err = proc.communicate()
s_return = proc.returncode
return s_return, s_output, s_err


3-Examples of HDFS commands from Python

Run Hadoop ls command in Python
(ret, out, err)= run_cmd(['hdfs', 'dfs', '-ls', 'hdfs_file_path'])
lines = out.split('\n')


Run Hadoop get command in Python
(ret, out, err)= run_cmd(['hdfs', 'dfs', '-get', 'hdfs_file_path', 'local_path'])


Run Hadoop put command in Python
(ret, out, err)= run_cmd(['hdfs', 'dfs', '-put', 'local_file', 'hdfs_file_path'])


Run Hadoop copyFromLocal command in Python
(ret, out, err)= run_cmd(['hdfs', 'dfs', '-copyFromLocal', 'local_file', 'hdfs_file_path'])

Run Hadoop copyToLocal command in Python
(ret, out, err)= run_cmd(['hdfs', 'dfs', '-copyToLocal', 'hdfs_file_path', 'local_file'])


hdfs dfs -rm -skipTrash /path/to/file/you/want/to/remove/permanently
Run Hadoop remove file command in Python
(ret, out, err)= run_cmd(['hdfs', 'dfs', '-rm', 'hdfs_file_path'])
(ret, out, err)= run_cmd(['hdfs', 'dfs', '-rm', '-skipTrash', 'hdfs_file_path'])


rm -r
HDFS Command to remove the entire directory and all of its content from HDFS.
Usage: hdfs dfs -rm -r <path>

(ret, out, err)= run_cmd(['hdfs', 'dfs', '-rm', '-r', 'hdfs_file_path'])
(ret, out, err)= run_cmd(['hdfs', 'dfs', '-rm', '-r', '-skipTrash', 'hdfs_file_path'])




Check if a file exist in HDFS
Usage: hadoop fs -test -[defsz] URI


Options:


-d: f the path is a directory, return 0.
-e: if the path exists, return 0.
-f: if the path is a file, return 0.
-s: if the path is not empty, return 0.
-z: if the file is zero length, return 0.
Example:


hadoop fs -test -e filename


hdfs_file_path = '/tmpo'
cmd = ['hdfs', 'dfs', '-test', '-e', hdfs_file_path]
ret, out, err = run_cmd(cmd)
print(ret, out, err)
if ret:
print('file does not exist')


These simple but very powerful lines of code allow to interact with HDFS in a programmatic way and can be easily scheduled as part of schedule cron jobs.