I understand what print does, but of what "type" is that language element? I think it's a function, but why does this fail?
>>> print print
SyntaxError: invalid syntax
Isn't print a function? Shouldn't it print something like this?
>>> print print
<function print at ...>
In 2.7 and down, print is a statement. In python 3, print is a function. To use the print function in Python 2.6 or 2.7, you can do
>>> from __future__ import print_function
>>> print(print)
<built-in function print>
See this section from the Python Language Reference, as well as PEP 3105 for why it changed.
In Python 3, print() is a built-in function (object)
Before this, print was a statement. Demonstration...
Python 2.x:
% pydoc2.6 print
The ``print`` statement
***********************
print_stmt ::= "print" ([expression ("," expression)* [","]]
| ">>" expression [("," expression)+ [","]])
``print`` evaluates each expression in turn and writes the resulting
object to standard output (see below). If an object is not a string,
it is first converted to a string using the rules for string
conversions. The (resulting or original) string is then written. A
space is written before each object is (converted and) written, unless
the output system believes it is positioned at the beginning of a
line. This is the case (1) when no characters have yet been written
to standard output, (2) when the last character written to standard
output is a whitespace character except ``' '``, or (3) when the last
write operation on standard output was not a ``print`` statement. (In
some cases it may be functional to write an empty string to standard
output for this reason.)
-----8<-----
Python 3.x:
% pydoc3.1 print
Help on built-in function print in module builtins:
print(...)
print(value, ..., sep=' ', end='\n', file=sys.stdout)
Prints the values to a stream, or to sys.stdout by default.
Optional keyword arguments:
file: a file-like object (stream); defaults to the current sys.stdout.
sep: string inserted between values, default a space.
end: string appended after the last value, default a newline.
print is a mistake that has been rectified in Python 3. In Python 3 it is a function. In Python 1.x and 2.x it is not a function, it is a special form like if or while, but unlike those two it is not a control structure.
So, I guess the most accurate thing to call it is a statement.
In Python all statements (except assignment) are expressed with reserved words, not addressible objects. That is why you cannot simply print print and you get a SyntaxError for trying. It's a reserved word, not an object.
Confusingly, you can have a variable named print. You can't address it in the normal way, but you can setattr(locals(), 'print', somevalue) and then print locals()['print'].
Other reserved words that might be desirable as variable names but are nonetheless verboten:
class
import
return
raise
except
try
pass
lambda
In Python 2, print is a statement, which is a whole different kind of thing from a variable or function. Statements are not Python objects that can be passed to type(); they're just part of the language itself, even more so than built-in functions. For example, you could do sum = 5 (even though you shouldn't), but you can't do print = 5 or if = 7 because print and if are statements.
In Python 3, the print statement was replaced with the print() function. So if you do type(print), it'll return <class 'builtin_function_or_method'>.
BONUS:
In Python 2.6+, you can put from __future__ import print_function at the top of your script (as the first line of code), and the print statement will be replaced with the print() function.
>>> # Python 2
>>> from __future__ import print_function
>>> type(print)
<type 'builtin_function_or_method'>
Related
I have a setup that looks as following:
from io import StringIO
from contextlib import redirect_stdout
f = StringIO()
with redirect_stdout(f):
exec("""'echo "test"'""")
s = f.getvalue()
print("return value:",s)
Is there a reason why the return value s does not contain "test"? How can I access this value?
exec executes Python code; it's not an equivalent of os.system.
What you are executing is the Python expression statement 'echo "test"', which simply evaluates to string. Expressions have values, but an expression statement ignores the value produced by the expression. Nothing is written to standard output in this case.
You want something like subprocess.check_output:
>>> subprocess.check_output("""echo 'test'""", shell=True).decode().rstrip('\n')
'test'
First argument to exec should be
The source may be a string representing one or more Python statements
or a code object as returned by compile()
you did
exec("""'echo "test"'""")
i.e. delivered to exec something which is not valid python statement(s).
So you can apply a variable amount of arguments from a list to a normal function by doing this
someFunc(*[1,2,3])
But it doesn't work when I used the built-in print
print(*[1,2,3])
^
SyntaxError: invalid syntax
How can I achieve the same effect with print?
The issue is that in python prior to python 3, print is a keyword of the language and not a function, so it doesn't work the same way. If you want to achieve a similar effect you can just make your own print function
def output(*args):
print ' '.join(str(i) for i in args)
then use output(*[1,2,3])
Or if your version of python2 is recent enough (>= 2.6) you can do
from __future__ import print_function
to get the python 3 semantics.
So I understand that if I do the following
print """ Anything I
type in here
works. Multiple LINES woohoo!"""
But what if following is my python script
""" This is my python Script. Just this much """
What does the above thing do? Is it taken as comment? Why is it not a syntax error?
Similarly, if I do
"This is my Python Script. Just this. Even with single quotes."
How are the above two scripts interpreted?
Thanks
The triple quotes ''' or """ are just different ways of representing strings. The advantage of triple quotes is that it can span multiple lines and sometimes serve as docstrings.
The reason:
"hadfasdfas"
doesn't raise any error is because python simply creates the string and then doesn't assign it to anything. For the python interpreter, it is perfectly fine if you have a pointless statement in your code as long as there are no syntax or semantics errors
Hope that helps.
The string is just evaluated, and the interpreter noticing it wasn't assigned to anything, throws it away.
But in some special places, this string is actually assigned to the __doc__ property of the item:
def func(arg):
"""
Does stuff. This string will be evaluated and assigned to func.__doc__.
"""
pass
class Test:
"""
Same for Test.__doc__
"""
pass
At the top of module.py:
"""
module does stuff. this will be assigned to module.__doc__
"""
def func():
...
In addition to #sshashank124 answer I have to add that triple quoted strings are also used in testing https://docs.python.org/2/library/doctest.html
So consider this code snippet:
def some_function(x, y):
"""This function should simply return sum of arguments.
It should throw an error if you pass string as argument
>>> some_function(5, 4)
9
>>> some_function(-5, 4)
-1
>>> some_function("abc", 4)
Traceback (most recent call last):
...
ValueError: arguments must numbers
"""
if type(x, str) or type(y, str):
raise ValueError("arguments must numbers")
else:
return x + y
if __name__ == "__main__":
import doctest
doctest.testmod()
If you import this tiny module, you'll get the some_function function.
But if you invoke this script directly from shell, tests given in the triple quoted string will be evaluated and the report will be printed to the output.
So triple quoted strings can be treated as values of type string, as comment, as docstrings and as containers for unittests.
In python everything is an object and you can pass it around easily.
So I can do :
>> def b():
....print "b"
>> a = b
>> a()
b
But if I do
a = print
I get SyntaxError . Why so ?
In Python 2.x, print is a statement not a function. In 2.6+ you can enable it to be a function within a given module using from __future__ import print_function. In Python 3.x it is a function that can be passed around.
In python2, print is a statement. If you do from __future__ import print_function, you can do as you described. In python3, what you tried works without any imports, since print was made a function.
This is covered in PEP3105
The other answers are correct. print is a statement, not a function in python2.x. What you have will work on python3. The only thing that I have to add is that if you want something that will work on python2 and python3, you can pass around sys.stdout.write. This doesn't write a newline (unlike print) -- it acts like any other file object.
print is not a function in pre 3.x python. It doesn't even look like one, you don't need to call it by (params)
Environment: python 2.x
If print is a built-in function, why does it not behave like other functions ? What is so special about print ?
-----------start session--------------
>>> ord 'a'
Exception : invalid syntax
>>> ord('a')
97
>>> print 'a'
a
>>> print('a')
a
>>> ord
<built-in function ord>
>>> print
-----------finish session--------------
The short answer is that in Python 2, print is not a function but a statement.
In all versions of Python, almost everything is an object. All objects have a type. We can discover an object's type by applying the type function to the object.
Using the interpreter we can see that the builtin functions sum and ord are exactly that in Python's type system:
>>> type(sum)
<type 'builtin_function_or_method'>
>>> type(ord)
<type 'builtin_function_or_method'>
But the following expression is not even valid Python:
>>> type(print)
SyntaxError: invalid syntax
This is because the name print itself is a keyword, like if or return. Keywords are not objects.
The more complete answer is that print can be either a statement or a function depending on the context.
In Python 3, print is no longer a statement but a function.
In Python 2, you can replace the print statement in a module with the equivalent of Python 3's print function by including this statement at the top of the module:
from __future__ import print_function
This special import is available only in Python 2.6 and above.
Refer to the documentation links in my answer for a more complete explanation.
print in Python versions below 3, is not a function. There's a separate print statement which is part of the language grammar. print is not an identifier. It's a keyword.
The deal is that print is built-in function only starting from python 3 branch. Looks like you are using python2.
Check out:
print "foo"; # Works in python2, not in python3
print("foo"); # Works in python3
print is more treated like a keyword than a function in python. The parser "knows" the special syntax of print (no parenthesis around the argument) and how to deal with it. I think the Python creator wanted to keep the syntax simple by doing so. As maverik already mentioned, in python3 print is being called like any other function and a syntx error is being thrown if you do it the old way.