I'm not a very experienced programmer, and I tried to find a library that contained a Python function that would print something only the first time this function was called from the same call and no more per script execution. Because I didn't find it, I created my own, and it works. But I don't know how to make it able to work just like print(f'') so I can include variables. Loops+break won't work here.
This is how it looks right now:
#dictionary so we don't have scope issues
already_printed = {}
def print_only_once(msg):
try:
testing = already_printed[msg]
except KeyError:
already_printed[msg] = msg
print(msg)
return
And I call it using:
print_only_once('my special message')
If this message has been printed before, it won't print again. I would like to be able to also do something like this:
print_only_once(f'temp is {var1} and humidity is {var2}')
And this special message would print only one time per script execution, regardless of how many times it was called, including with differente values for var1 and/or var2 from the first call.
Any help? Maybe there's something that does exactly that out there but I couldn't find it. Thanks a lot!
You can't do this when you format the string in the caller, because your function only receives the formatted string.
You need to have your function take the format string and values separately. Then you can save the format string in your dictionary, and call .format() explicitly.
already_printed = set()
def print_only_once(format_string, *args, **kwargs):
if format_string not in already_printed:
print(format_string.format(*args, **kwargs))
already_printed.add(format_string)
print_only_once('temp is {var1} and humidity is {var2}', var1=var1, var2=var2)
Try doing something like this:
already_printed = []
def print_only_once(msg_name, msg):
if msg_name not in already_printed:
already_printed.append(msg_name)
print(msg)
return
I added another parameter called "msg_name". You should write what the message is about in there (like giving a group name). And you write your actual text/message in "msg".
Related
So I have a function that returns a value. I'd like it to write it into shell only when called with print.
So c.function() doesn't return anything, while print(c.function) returns the value I need
Edit: I probably worded the question wrong
I have a class function that goes over an iterable, performs different tasks based on elements of the iterable. After completing the tasks it returns a number of the tasks completed. I need the function to write into shell only when called as print(c.function(iterable)), and to show nothing when called as c.function(iterable), is that possible?
Thanks in advance
If you write a function like,
def foo():
a = "This works."
return a
then call the function in the script like
foo()
nothing gets printed.
However, if you assign the function return call to a variable like
b = foo()
and print(b), then you get the result printed.
Simply assign to a dummy variable when you want to hide the output:
>>> _ = c.function()
And call the function normally when you want to view the output:
>>> c.function()
'the value you need'
Just in case you need to view the result, you can:
>>> _
'the value you need'
Although _ is also used in some Python shells as a special variable for the last value evaluated, so this might not always work. You might be able to rig up a fancy solution by assigning to a simple name that points to a property that appends to a list, but that seems like a lot of effort.
FWIW IPython can be made to not echo the value with a semicolon (e.g. 1; doesn't print anything) or can be set to not automatically echo values plus it automatically records every input and output.
I have several pre-defined functions in a dictionary:
dict = {
'test1':test1(),
'test2':test2(),
'test3':test3()
}
At this point I already have a question: after creating the dictionary these functions run automatically - why is that and how can I avoid it?
Ultimately, my goal is to type in e.g. "test2" and get the function executed (just the one in a controlled manner, not everything at once like above). First I wanted to use eval, but everyone advised against it, which I understand now. But what is the alternative? I tried
def select_function():
try:
return dict[input("Type in function ")]
except KeyError:
raise ValueError('Invalid input!')
select_function()
as well as just
dict.get(input("Type in function "), 'Invalid input!')
But both return an empty line if I type in the correct function. If I type in a wrong one I get my error message, therefore my command should work in general. I also know my functions work because, as I said, they get executed after I create the dictionary and they only print a greeting (a different one for each function for testing).
Did I do something wrong? Is there some other way to call functions without eval()?Thanks in advance.
Do it like
dict = {
'test1':test1,
'test2':test2,
'test3':test3
}
Then
return dict[input("Type in function")]
If you want to call that function, Just
return dict[input("Type in function")]()
I am trying to make a function's output behave as if it's my input. The goal is to make a new output from the old output.
I have some code that looks like this:
def func():
BLOCK OF CODE
func()
There is no return statement in the function and no parameters within the parenthesis.
When I type func() to call my function as shown above, I get the desired output, which is a bunch of printed statements. Now I want to do something with that output to get another output.
All I'm trying to do is effectively "pipe" the output of one function into the input of another function (or, if possible, not even worry about creating another function at all, and instead doing something more direct). I looked into Python 3 writing to a pipe
but it did not help me. I also tried defining another function and using the preceding function as a parameter, which did not work either:
def another_func(func):
print another_statement
another_func(func)
I also tried making a closure (which "kind" of worked because at least it printed the same thing that func() would print, but still not very encouraging):
def func():
def another_func():
print another_statement
BLOCK OF CODE
another_func()
Finally, I tried designing both a decorator and a nested function to accomplish this, but I have no parameters in my function, which really threw off my code (didn't print anything at all).
Any advice on how to manipulate a function's output like as if it is your input so that it's possible to create a new output?
You could achieve this by redirecting stdout using a decorator:
from StringIO import StringIO
import sys
def pipe(f):
def decorated(*args, **kwargs):
old,sys.stdout = sys.stdout,StringIO()
try:
result = f(*args, **kwargs)
output = sys.stdout.getvalue()
finally:
sys.stdout = old
return result, output
return decorated
You could then get the result, output pair from any decorated function, eg:
#pipe
def test(x):
print x
return 0
test(3) -> (0, '3\n')
However, I can't think of a good reason why you'd want to do this.
(Actually, that's not quite true; it is handy when writing unit tests for user IO, such as when testing student assignments in a software engineering course. I seriously doubt that that's what the OP is trying to do, though.)
Return the desired value(s) from the function - instead of printing the values on the console, return them as strings, numbers, lists or any other type that makes sense. Otherwise, how do you expect to "connect" the output of a function as the input to another, if there is no output to begin with?
Of course, printing on the console doesn't count as output unless you're planning to eventually use OS pipes or a similar mechanism to connect two programs on the console, but keep things simple! just use the function's return values and worry about pipes later if and only if that's necessary for your problem in particular.
After reading the comments: "connecting" two functions by printing on the console from one and reading from the console from the other would be a really bad idea in this case, first you have to grasp the way functions return values to each other, trust me on this one: you have to rethink your program! even though other answers (strictly speaking) answer your original question, that's absolutely not what you should do.
just for fun ... because OP asked for it
import StringIO
import sys
def func1():
for i in range(1,10):
print "some stuff %d"%i
def func2(func):
old_std = sys.stdout
sys.stdout = StringIO.StringIO()
try:
func()
return sys.stdout.getvalue().splitlines()
finally:
sys.stdout = old_std
print func2(func1)
You need to return a value from your function. This can be used to assign the value into another variable.
Say I define some function doubleThis that will double the input
def doubleThis(x):
print 'this is x :', x
return x * 2 # note the return keyword
Now I can call the function with 3, and it returns 6 as expected
>>> doubleThis(3)
this is x : 3
6
Now I have another function subtractOne that returns the input value, minus 1.
def subtractOne(i):
print 'this is i :', i
return i - 1
Now comes the answer to your question. Note that we can call the first function as the input to the second, due to the fact that it has a return value.
>>> subtractOne(doubleThis(3))
this is x : 3
this is i : 6
5
Being new at programming in general, and new with Python in particular, I'm having some beginner's troubles.
I'm trying out a function from NLTK called generate:
string.generate()
It returns what seems like a string. However, if I write:
stringvariable = string.generate()
or
stringvariable = str(string.generate())
… the stringvariable is always Empty.
So I guess I'm missing something here. Can the text output generated, that I see on the screen, be something else than a string output? And if so, is there any way for me to grab that output and put it into a variable?
Briefly put, how to I get what comes out of string.generate() into stringvariable, if not as described above?
you can rewrite generate. The only disadvantage is that it can change and your code might not be updated to reflect these changes:
from nltk.util import tokenwrap
def generate_no_stdout(self, length=100):
if '_trigram_model' not in self.__dict__:
estimator = lambda fdist, bins: LidstoneProbDist(fdist, 0.2)
self._trigram_model = NgramModel(3, self, estimator=estimator)
text = self._trigram_model.generate(length)
return tokenwrap(text)
then "a.generate()" becomes "generate_no_stdout(a)"
generate() prints its output rather than returning a string, so you need to capture it.
I'm currently working on an experiment where I'm implementing an interpreter for an old in-game scripting language. It's a forth based language, so I figure it would be fairly easy to just have the instructions (once verified and santized) put into a big list.
Once I've got the code in a list, I am trying to iterate through the entire program in a for loop that processes the instructions one at a time. Certain items, like strings, could be placed onto a variable that holds the current stack, which is easy enough. But where I'm stuck is making commands happen.
I have a big list of functions that are valid and I'd like it to where if any instruction matches them, it calls the associated function.
So, for example, if I had:
"Hello, world!" notify
...the code would check for notify in a list and then execute the notify function. The bottom line is: How do I translate a string into a function name?
You could keep a dictionary of functions the code can call, and then do a look up when you need to:
def notify(s):
print(s)
d = {"notify": notify}
d["notify"]("Hello, world!")
You can do it through locals which is a dictionary with th current local symbol table:
locals()["notify"]()
or though globals which returns a dictionary with the symbol table of globals:
globals()["notify"]()
You can give arguments too e.g.:
locals()["notify"]("Hello, world!")
or
globals()["notify"]("Hello, world!")
If you have a dict called commands that maps names to functions, you can do it like this:
def my_notify_function():
print(stack.pop)
commands = {'notify': my_notify_function, ...}
for item in program:
if item in commands:
commands[item]()
else:
stack.push(item)
Something like:
import re
class LangLib(object):
pattern = re.compile(r'"(.*?)" (.*)')
def run_line(self, line):
arg, command = re.match(LangLib.pattern, line).groups()
return getattr(self, command)(arg)
def notify(self, arg):
print arg
Then your engine code would be:
parser = LangLib()
for line in program_lines:
parser.run_line(line)
Create a dictionary of function names and some tags.
I have tried it several times before, it works really well.