I would assume it's due to buffering. Try adding flush=True as one of the optional parameter to print.

You can use enumerate to get that serial numbers starting from 1.

To print the output the way you mentioned you need to use String formatting - Docs

I have used {item:25} - This will make the item to be of width 25. You can put whatever number you wish to get desired space.

From the Docs:

Passing an integer after the ':' will cause that field to be a minimum number of characters wide. This is useful for making columns line up.

with open('menu.txt', 'r') as f:
    lines = f.readlines()
    for i,v in enumerate(lines,1):
        item, price = v.strip('\n').split(',')
        print(f'{i}. {item:25}{price}')

Output

1. Cheese Burger             5.00
2. Chicken Burger            4.50
3. Fries                     2.00
4. Onion Rings               2.50
5. Drinks                    1.50

Python does not automatically call main() (and you'll need to use the sys library to get argv).

#!/usr/bin/python

import sys

def main():
    print  "hell0\n"

main()

In Python 3, print became a function. This means that you need to include parenthesis now like mentioned below:

print("Hello World")

Add the parenthesis:

print('Output After Training:')

If you need compatibility for code so works with Python 2 or 3, use:

from __future__ import print_function

at the start of the code file and then use Python 3 way of using print().

Learn more about print() in Python 3 at:

• https://www.python-course.eu/python3_print.php

f.read() reads the entire file and leaves the current position at the end of the file, so the 2nd f.read() call starts from the end of the file and doesn't read anything. Instead, save the read content to a variable.

In addition, w+ truncates the file first, use r+ to avoid this.

I'm also assuming that you want to clear the previous value in the file, so use f.truncate to clear the file and f.seek to reset the position to the start of the file.

Putting it all together:

with open("count.txt","r+") as f:
    content = f.read()
    print(content)
    a = int(content)
    b = a+1
    print(b)
    f.truncate(0)
    f.seek(0)
    f.write(str(b))
input()

In Python3 print is a function:

print("Hello, World!")

Check: http://docs.python.org/release/3.0.1/whatsnew/3.0.html

Python is whitespace sensitive, but docstrings may prevent you from getting a proper error due to your indentation.

Looks like your code should be indented like this:

au = 1.49598e11                        #astronomical unit in meters
rx = au * np.asarray([.5,.8,.2])       #x-comp separation vector
ry = au * np.asarray([2.6,9.1,3.7])    #y-comp separation vector
rz = au * np.asarray([.05,.1,.25])     #z-comp separation vector
def svec(x,y,z):
    '''computes the magnitude and vector components of the distance between two particles.'''

    #for loop to compute vector components of separation between two particles
    rvec = []
    for i in range(3):
        if i < 2:
            vx = x[i]-x[i+1]
            vy = y[i]-y[i+1]
            vz = z[i]-z[i+1]
            rvec.append([vx,vy,vz])
        if i == 2:
            vx = x[0]-x[-1]
            vy = y[0]-y[-1]
            vz = z[0]-z[-1]
            rvec.append([vx,vy,vz])
    return rvec

comp1 = ['x-comp[m]','y-comp[m]','z-comp[m]']
r0 = rvec[0].insert(0,'particle 0->1')
r1 = rvec[1].insert(0,'particle 1->2')
r2 = rvec[2].insert(0,'particle 0->2')
print(tabulate(rvec,headers=comp1))

You are almost certainly experiencing output buffering by line. The output buffer is flushed every time you complete a line, but by suppressing the newline you never fill the buffer far enough to force a flush.

You can force a flush using flush=True (Python 3.3 and up) or by calling the flush() method on sys.stdout:

for entry in categories:
    print(entry, ", ", end='', flush=True)

You could simplify that a little, make , the end value:

for entry in categories:
    print(entry, end=', ', flush=True)

to eliminate the space between the entry and the comma.

Alternatively, print the categories as one string by using the comma as the sep separator argument:

print(*categories, sep=', ')