Learning Python - Mark Lutz [158]
Calls and other expressions
Running functions
log.write("spam, ham")
print calls
Printing objects
print('The Killer', joke)
if/elif/else
Selecting actions
if "python" in text:
print(text)
for/else
Sequence iteration
for x in mylist:
print(x)
while/else
General loops
while X > Y:
print('hello')
pass
Empty placeholder
while True:
pass
break
Loop exit
while True:
if exittest(): break
continue
Loop continue
while True:
if skiptest(): continue
def
Functions and methods
def f(a, b, c=1, *d):
print(a+b+c+d[0])
return
Functions results
def f(a, b, c=1, *d):
return a+b+c+d[0]
yield
Generator functions
def gen(n):
for i in n: yield i*2
global
Namespaces
x = 'old'
def function():
global x, y; x = 'new'
nonlocal
Namespaces (3.0+)
def outer():
x = 'old'
def function():
nonlocal x; x = 'new'
import
Module access
import sys
from
Attribute access
from sys import stdin
class
Building objects
class Subclass(Superclass):
staticData = []
def method(self): pass
try/except/ finally
Catching exceptions
try:
action()
except:
print('action error')
raise
Triggering exceptions
raise EndSearch(location)
assert
Debugging checks
assert X > Y, 'X too small'
with/as
Context managers (2.6+)
with open('data') as myfile:
process(myfile)
del
Deleting references
del data[k]
del data[i:j]
del obj.attr
del variable
Table 10-1 reflects the statement forms in Python 3.0—units of code that each have a specific syntax and purpose. Here are a few fine points about its content:
Assignment statements come in a variety of syntax flavors, described in Chapter 11: basic, sequence, augmented, and more.
print is technically neither a reserved word nor a statement in 3.0, but a built-in function call; because it will nearly always be run as an expression statement, though (that is, on a line by itself), it’s generally thought of as a statement type. We’ll study print operations in Chapter 11 the next chapter.
yield is actually an expression instead of a statement too, as of 2.5; like print, it’s typically used in a line by itself and so is included in this table, but scripts occasionally assign or otherwise use its result, as we’ll see in Chapter 20. As an expression, yield is also a reserved word, unlike print.
Most of this table applies to Python 2.6, too, except where it doesn’t—if you are using Python 2.6 or older, here are a few notes for your Python, too:
In 2.6, nonlocal is not available; as we’ll see in Chapter 17, there are alternative ways to achieve this statement’s writeable state-retention effect.
In 2.6, print is a statement instead of a built-in function call, with specific syntax covered in Chapter 11.
In 2.6, the 3.0 exec code execution built-in function is a statement, with specific syntax; since it supports enclosing parentheses, though, you can generally use its 3.0 call form in 2.6 code.
In 2.5, the try/except and try/finally statements were merged: the two were formerly separate statements, but we can now say both except and finally in the same try statement.
In 2.5, with/as is an optional extension, and it is not available unless you explicitly turn it on by running the statement from __future__ import with_statement (see Chapter 33).
A Tale of Two ifs
Before we delve into the details of any of the concrete statements in Table 10-1, I want to begin our look at Python statement syntax by showing you what you are not going to type in Python code so you can compare and contrast it with other syntax models you might have seen in the past.
Consider the following if statement, coded in a C-like language:
if (x > y) {
x = 1;
y = 2;
}
This might be a statement in C, C++, Java, JavaScript, or Perl. Now, look at the equivalent statement in the Python language:
if x > y:
x = 1
y = 2
The first thing that may pop out at you is that