How to call a Python script with arguments from Java class

Question

I am using Python 3.4.

I have a Python script myscript.py :

import sys
def returnvalue(str) :
 if str == "hi" :
 return "yes"
 else :
 return "no"
print("calling python function with parameters:")
print(sys.argv[1])
str = sys.argv[1]
res = returnvalue(str)
target = open("file.txt", 'w')
target.write(res)
target.close()

I need to call this python script from the java class PythonJava.java

public class PythonJava 
{
 String arg1;
 public void setArg1(String arg1) {
 this.arg1 = arg1;
 }
public void runPython() 
 { //need to call myscript.py and also pass arg1 as its arguments.
 //and also myscript.py path is in C:\Demo\myscript.py
}

and I am calling runPython() from another Java class by creating an object of PythonJava

obj.setArg1("hi");
...
obj.runPython();

I have tried many ways but none of them are properly working. I used Jython and also ProcessBuilder but the script was not write into file.txt. Can you suggest a way to properly implement this?

• cdarke: Bad idea to call a variable (in Python) str because it will mask the system string class of the same name.

Answer 1

Have you looked at these? They suggest different ways of doing this:

Call Python code from Java by passing parameters and results

How to call a python method from a java class?

In short one solution could be:

public void runPython() 
{ //need to call myscript.py and also pass arg1 as its arguments.
 //and also myscript.py path is in C:\Demo\myscript.py
 String[] cmd = {
 "python",
 "C:/Demo/myscript.py",
 this.arg1,
 };
 Runtime.getRuntime().exec(cmd);
}

edit: just make sure you change the variable name from str to something else, as noted by cdarke

Your python code (change str to something else, e.g. arg and specify a path for file):

def returnvalue(arg) :
 if arg == "hi" :
 return "yes"
 return "no"
print("calling python function with parameters:")
print(sys.argv[1])
arg = sys.argv[1]
res = returnvalue(arg)
print(res)
with open("C:/path/to/where/you/want/file.txt", 'w') as target: # specify path or else it will be created where you run your java code
 target.write(res)

• Sushin K.Kumar: i have changed the variable name and when i run the program i am getting the following error : java.io.IOException: Cannot run program "python C:/Demo/myscript.py": CreateProcess error=2, The system cannot find the file specified Can you help me with this?
• mkaran: is C:/Demo/myscript.py the right path? if yes try C:\\Demo\\myscript.py even though forward slashes should not be an issue. I'll check it locally in a minute.
• mkaran: Please check the edited answer, each command/ argument needs to be separate, my bad! I tried it and it works fine.
• Sushin K.Kumar: same error if i am using C:\\Demo\\myscript.py and also the file exists in that path. Please find the error below java.io.IOException: Cannot run program "python C:\Demo\myscript.py": CreateProcess error=2, The system cannot find the file specified
• mkaran: Check the updated answer :) String[] cmd = { "python", // separate "C:/Demo/myscript.py", // separate this.arg1, };

Answer 2

calling python from java with Argument and print python output in java console can be done with below simple method:

String pathPython = "pathtopython\\script.py";
String [] cmd = new String[3];
cmd[0] = "python";
cmd[1] = pathPython;
cmd[2] = arg1;
Runtime r = Runtime.getRuntime();
Process p = r.exec(cmd);
BufferedReader in = new BufferedReader(new InputStreamReader(p.getInputStream()));
while((s = in.readLine()) != null){
 System.out.println(s);
 }

Answer 3

Below is the python method with three sample arguments which can later be called through java.

#Sample python method with arguments
import sys
 
def getDataFromJava(arg1,arg2,arg3):
 
 arg1_val="Hi"+arg1
 arg2_val=arg2
 arg3_val=arg3
 
 print(arg1_val)
 print(arg2_val)
 print(arg3_val)
 
 return arg1_val,arg2_val,arg3_val
 
 
 
arg1 = sys.argv[1]
arg2 = sys.argv[2]
arg3 = sys.argv[3]
getDataFromJava(arg1,arg2,arg3)

Below is the java code to invoke the above method with three sample arguments and also read console output of python script in java through InputStreamReader.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class Test1 {
 
 public static String s;
 
 public static void main(String[] args) throws IOException {
 
 
 
 String pathPython = "Path_to_the_file\\test.py";
 String [] cmd = new String[5];
 cmd[0] = "python";
 cmd[1] = pathPython;
 cmd[2] = "arg1";
 cmd[3] = "arg2";
 cmd[4] = "arg3";
 Runtime r = Runtime.getRuntime();
 
 Process p = r.exec(cmd);
 BufferedReader in = new BufferedReader(new InputStreamReader(p.getInputStream()));
 while((s=in.readLine()) != null){
 System.out.println(s);
 }
 
 
 }
}