Programming Statement

Python Parser supports the if-else statement and the if-elif-else statement.

x = 10
if x == 0:
    print("Zero")
elif x > 0:
    print("Positive")
else:
    print("Negative")

The for statement is used to iterate over iterable sequences, such as strings, lists, tuples, dictionaries (dict.items()), and sets.

• Use the for statement to iterate over a range:

  for i in range(3):
  	print(i)

  Output:

  0

  1

  2

• Use the for statement to iterate over a string:

  for item in "ddb":
  	print(item)

  Output:

  d

  d

  b

• Use the for statement to iterate over a list:

  words = ['cat', 'window', 'defenestrate']
  for w in words:
      print(w, len(w))

  Output:

  cat 3

  window 6

  defenestrate 12

• Use the for statement to iterate over a dictionary:

  dict_1 = {'Name': 'Zara', 'Age': 7, 'Class': 'First','Address':'Beijing'}
  for key in dict_1:
      print(key, ":", dict_1[key])

  Output:

  Name : Zara

  Age : 7

  Class : First

  Address : Beijing

• Use the for statement to iterate over a set:

  A = {'1','2','star'}
  for item in A:
      print(item,end='')

  Output:

  1

  2

  star

i = 1
while i<=10:
	
	if i == 5:
		break		
	print(i)	
	i+=1

Output:

1

2

3

4

As keywords, break and continue can only be used within loops. The break statement is used to terminate a loop, while the continue statement is used to skip the current iteration of a loop but continue with the next.

for i in range(1, 6):
    if i == 4:
        print('finish')
        break  # Terminate a loop.
    print('I am ' + str(i) + ' years old')

Output:

I am 1 years old

I am 2 years old

I am 3 years old

finish

for i in range(1, 6):
    if i == 4:
        print('finish')
        continue  # Skip the current iteration of a loop and continue with the next.
    print('I am ' + str(i) + ' years old')

Output:

I am 1 years old

I am 2 years old

I am 3 years old

finish

I am 5 years old

The pass statement is used as a placeholder to keep the output well-formed.

for letter in 'pear':
   if letter == 'a':
      pass
      print ('This is pass block')
   print ('Current Letter :', letter)

print ("Good bye!")

Output:

Current Letter : p

Current Letter : e

This is pass block

Current Letter : a

Current Letter : r

Good bye!

first = [2, 3, 4]
second = [20, 30, 40]
final = []
for i in first:
    for j in second:
        final.append(i+j)
print(final)

Output: [22, 32, 42, 23, 33, 43, 24, 34, 44]

The assert statement in DolphinDB supports assert expression and assert string, expression, while the assert statement in Python Parser supports assert expression and assert expression [, arguments].

assert 1==1
assert 1==2, '1 is not equal to 2'

Output: Testing case adhocTesting_"1 is not equal to 2" failed

The try-catch statement in the current version of Python Parser can only catch all exceptions.

def this_fails():
    x = 1/`7

try:
    this_fails()
except Exception as e:
    print('error occur:' + e)

Output: "error occur:SYSTEM_Operator" : "error occur:Both arguments for ratio must be numbers."

To raise a user-defined exception, DolphinDB uses the throw statement while Python Parser uses the raise statement with the syntax raise expression.

def test_exception():
    try:
            raise "asdf"
    except Exception as ex:
            assert ex[0]=="USER"
            assert ex[1]=="asdf"
            assert typestr(ex) == "STRING PAIR"
    try:
            assert 1 == 112
            1 + 2
    except Exception as ex:
            pass
            a = 123
            pass
            pass
            assert ex[0]=="C++"
            assert ex[1]=="Testing case adhocTesting failed"
    print("test_exception test pass")

test_exception()

Output: test_exception test pass

The comments in Python are different from those in DolphinDB but the same as those in Python.

Single-line comments start with the hash symbol # as follows:

// This is a comment
print("Hello, World!")

Multiline comments are enclosed with triple quotes (''' or """) as follows:

'''
This is the first comment
This is the second comment 
This is the third comment
'''
print("Hello, World!")

In DolphinDB, module files use file extension ".dos" and stored in "<DolphinDB Home>/modules". In a DolphinDB session, users can reference a module with the "use" keyword.

In Python Parser, module files use file extension ".py" and stored in "<DolphinDB Home>/python" or a user-defined path (defined by sys.path). To use a user-defined path, it is necessary to restart the server before importing modules in Python Parser sessions.

Create a module file mod_test.py in "<DolphinDB Home>/python":

def f():
  a = 10
  b = 15
  return a + b

To import a module, use the import statement:

import mod_test
mod_test.f()

Output: 16

To import only parts from a module, use the from..import keyword:

from mod_test import f
f()

Output: 16

To relocate a module, use sys.path to specify the search path for a module:

import sys
sys.path.append("/path/to/module")

To remove a module, use the undef statement:

undef("mod_test")