Variables¶

There are several types of variables:

a = True                      # Boolean
b = False                     # Boolean

c = 123                       # Integer, range is -32768 to 32767
d = "hello"                   # String, size limits vary by platform
e = (None, True, 2, "Three")  # Tuple – usable only as a constant in SNAPpy

f = None                      # Python has a "None" data type
g = startup                   # Function
h = xrange(0, 10, 3)          # Iterator (introduced in SNAP 2.6)
i = [1, 1, 2, 3, 5, 8]        # Byte List (introduced in SNAP 2.6)

In the above example, invoking g() would be the same as directly calling startup(). You can use the type() function to determine the type of any variable in SNAPpy.

Variables can change their type on the fly:

x = 99              # variable x is currently an integer (int)
x = False           # variable x is now a Boolean value of False
x = "hello"         # variable x is now a string (str)
x = (x == "hello")  # variable x is now a Boolean value of True

String variables can contain binary data:

A = "\x00\xFF\xAA\x55"  # The "\x" prefix means hexadecimal character
B = "Pi\xe1"            # This creates a string of length 3

Byte lists allow for updates without rebuilding:

A = [7, 8, 9]
A[2] += 1

Variables at the top of your script are global:

x = 99  # this is a global variable

def sayHello():
    print("x=", x)

Variables within functions are usually local:

x = 99  # this is a global variable

def showNumber():
    x = 123  # this is a separate local variable
    print(x)  # prints 123

Unless you explicitly say you mean the global one:

x = 99  # this is a global variable

def showGlobal():
    print(x)  # this shows the current value of global variable x

def changeGlobal():
    global x # because of this statement
    x = 314  # this changes the global variable x

def changeLocal():
    x = 42   # this statement does not change the global variable x
    print(x)  # will print 42 but the global variable x is unchanged

Creating globals on the fly:

def newGlobal():
    global x   # this is a global variable, even without previous declaration
    x = x + 1  # ERROR! - variables must be initialized before use

    if x > 7:  # ERROR! – variables must be initialized before use
        pass

These two statements are not errors if some other function has previously initialized a value for global variable x before the newGlobal() function runs. Globals declared in this way have the same availability as globals explicitly initialized outside the scope of any function.

Note

On RAM-constrained devices, SNAPpy scripts limit the number of concurrent local variables and system global variables. See SNAP Modules for more information on limits.

Functions

Operators