🌐 Mailman 3 November 2018 - Tutor - python.org

Scripting Calligra sheets with Python
by Jim Byrnes Jan. 31, 2022

Jan. 31, 2022

I am trying to script Calligra Sheets (formerly KSpread) with python. I have gotten some of the included example scripts to run so I know python scripting is running. I found the following snippet on their website: import KSpread sheet = KSpread.view().sheet() # swap text of B5 and C6 t1 = sheet.text("B5") t2 = sheet.text(6,3) sheet.setText("B5", t2) sheet.setText(6, 3, t1) # swap value of D7 and E8 v1 = sheet.value("D7") v2 = sheet.value(8,5) sheet.setValue("D7", v2) sheet.setValue(8, 5, v1) Error: 'str' object has no attribute 'text' File "file:///usr/share/kde4/apps/sheets/scripts/extensions/myswap.py", line 4, in <module> This error message appeared in a dialog box. I've taught myself some Python but I don't understand what is happening here. Shouldn't t1 be a sheet object? What would cause it to be a str instead? Thanks, Jim

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look back comprehensively
by Avi Gross Dec. 27, 2018

Dec. 27, 2018

I have been thinking about the thread we have had where the job seemed to be to read in a log file and if some string was found, process the line before it and generate some report. Is that generally correct? The questioner suggested they needed both the entire file as one string but also as a list of strings. Suggestions were made, IF SO, to read the entire file twice, once as a whole and once as lines. Another suggestion was to read either version ONCE and use cheaper Python methods to make the second copy. We also looked at a similar issue about using a buffer to keep the last N lines. I thought of another tack that may be in between but allow serious functionality. OUTLINE: Use just the readlines version to get a list of strings representing each line. Assuming the searched text is static, no need for regular expressions. You can ask if 'something' in line But if you find it, you may not have an index so a use of enumerate (or zip) to make a tuple might be of use. You can do a list comprehension on an enumerate object to get both the indexes where the requested 'something' was found and also optionally the line contents (ignored) and be able to use the indices to look at the index (or more) before. Here is an example using a fake program where I create the four lines and generate only the tuples needed for further processing, or just the indices for the line ABOVE where it was found. It can be done with simple list comprehensions or made into a generator expression. -CODE- """ Sample code showing how to read a (simulated) file and search for a fixed string and return the item number in a list of strings for further processing including of earlier lines. """ # Make test data without use of file fromfile = str1 = """alpha line one beta line two gamma line three alphabet line four""" lines= fromfile.split('\n') print("RAW data: ", lines) # just for illustration errors = [(index,line) for (index, line) in enumerate(lines) if 'bet' in line] just_indices = [index - 1 for (index, line) in enumerate(lines) if 'bet' in line] from pprint import pprint print("ERROR tuples:") # just for illustration pprint(errors) print("Just error indices:") pprint(just_indices) -END-CODE- -OUTPUT- RAW data: ['alpha line one', 'beta line two', 'gamma line three', 'alphabet line four'] ERROR tuples: [(1, 'beta line two'), (3, 'alphabet line four')] Just error indices: [0, 2] -END-OUTPUT- Again, this did two ways, and only one is needed. But the next step would be to iterate over the results and process the earlier line to find whatever it is you need to report. Many ways to do that such as: for (index, ignore) in errors or for index in just_indices You can use a regular expression on a line at a time. And so on. Again, other methods mentioned work fine, and using a deque to store earlier lines in a limited buffer while not reading the entire file into memory would also be a good way. Warning: the above assumes the text found will never be in the zeroeth line. Otherwise, you need to check as accessing line -1 may actually return the last line! As stated many times, there seem to be an amazing number of ways to do anything. As an example, I mentioned using zip above. One obvious method is to zip it with a range statement making it look just like enumerate. A more subtle one would be to make a copy of the set of lines the same length but each line content shifted by one. Zip that to the original and you get a tuple with (line 0, null) then (line 1, line 0) up to (line n, line n-1) Yes, that doubles memory use but you can solve so much more in one somewhat more complicated list comprehension. Anyone want to guess how? If you recall, some regular expression matches something on the previous line. Let us make believe you wrote a function called do_the_match(line, re) that applies the regular expression to the line of text and returns the matching text or perhaps an empty string if not found. So if you define that function then the following code will work. First, make my funny zip: I make lines_after as copy of lines shifted over one. Or more exactly circularly permuted by one. The goal is to search in line_after and if found, do the regular expression match in line before. >>> lines_after = lines[1:] >>> lines_after.append(lines[0]) >>> lines_after ['beta line two', 'gamma line three', 'alphabet line four', 'alpha line one'] >>> lines ['alpha line one', 'beta line two', 'gamma line three', 'alphabet line four'] >>> list(zip(lines_after, lines)) [('beta line two', 'alpha line one'), ('gamma line three', 'beta line two'), ('alphabet line four', 'gamma line three'), ('alpha line one', 'alphabet line four')] So the list comprehension looks something like this: matches = [ do_theMatch(line, re) for (line_after, line) in zip(line_after, line) if 'something' in line_after ] Need I mention the above code was used in Python 3.7.0 ??? I think it is important to try to learn the idioms and odd customs of a language even if sometimes it results in more memory or CPU usage but maybe better not to overly complicate the code till nobody understands it. The latter may be in a mild way 'elegant' but even if I made the variable names more descriptive it might be harder to understand than a simple enumerated version or several passes in more normal iteration statements. Feel free to comment. I have a thick skin and love to learn from others. Avi

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I need help with my project
by Treyton Hendrix Dec. 3, 2018

Dec. 3, 2018

If the user selected a sandwich, french fries, and a beverage, reduce the total cost of the order by $1.00. This is what I have to do and I don't know where to start.

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Moving a conda environment to an off-line computer
by Henrique Castro Dec. 3, 2018

Dec. 3, 2018

Dear colleagues, Soon I'll start to use one of the powerful computers on my university as a tool in my Ph.D. The computer does not have an internet connection and I need to find a way to install a conda environment on it. At first I tried to install and set the conda environment that I need in a computer with internet connection and taking care to copy everything in a path that is similar in the off-line computer (i.e I installed everything on /home/henrique/bin/anaconda3 at home and tried to copy everything to /home/henrique/bin/anaconda3 in the off-line computer - with the same .bashrc) but when I run conda I get an error(it works on my home computer): (deepchem) [henrique@europio qm7] $ python qm7_ANI.py /home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/site-packages/sklearn/ensemble/weight_boosting.py:29: DeprecationWarning: numpy.core.umath_tests is an internal NumPy module and should not be imported. It will be removed in a future NumPy release. from numpy.core.umath_tests import inner1d Traceback (most recent call last): File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/urllib/request.py", line 1318, in do_open encode_chunked=req.has_header('Transfer-encoding')) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/http/client.py", line 1239, in request self._send_request(method, url, body, headers, encode_chunked) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/http/client.py", line 1285, in _send_request self.endheaders(body, encode_chunked=encode_chunked) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/http/client.py", line 1234, in endheaders self._send_output(message_body, encode_chunked=encode_chunked) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/http/client.py", line 1026, in _send_output self.send(msg) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/http/client.py", line 964, in send self.connect() File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/http/client.py", line 936, in connect (self.host,self.port), self.timeout, self.source_address) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/socket.py", line 704, in create_connection for res in getaddrinfo(host, port, 0, SOCK_STREAM): File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/socket.py", line 745, in getaddrinfo for res in _socket.getaddrinfo(host, port, family, type, proto, flags): socket.gaierror: [Errno -2] Name or service not known During handling of the above exception, another exception occurred: Traceback (most recent call last): File "qm7_ANI.py", line 15, in <module> featurizer='BPSymmetryFunction', split='stratified', move_mean=False) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/site-packages/deepchem/molnet/load_function/qm7_datasets.py", line 50, in load_qm7_from_mat 'http://deepchem.io.s3-website-us-west-1.amazonaws.com/datasets/qm7.mat' File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/site-packages/deepchem/utils/__init__.py", line 85, in download_url urlretrieve(url, os.path.join(dest_dir, name)) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/urllib/request.py", line 248, in urlretrieve with contextlib.closing(urlopen(url, data)) as fp: File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/urllib/request.py", line 223, in urlopen return opener.open(url, data, timeout) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/urllib/request.py", line 526, in open response = self._open(req, data) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/urllib/request.py", line 544, in _open '_open', req) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/urllib/request.py", line 504, in _call_chain result = func(*args) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/urllib/request.py", line 1346, in http_open return self.do_open(http.client.HTTPConnection, req) File "/home/henrique/bin/anaconda3/envs/deepchem/lib/python3.6/urllib/request.py", line 1320, in do_open raise URLError(err) urllib.error.URLError: <urlopen error [Errno -2] Name or service not known> Any help is much appreciated -- Henrique C. S. Junior

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Search error code in a logfile and print all lines until the error code
by Asad Dec. 1, 2018

Dec. 1, 2018

Hi All , I need some help to print error lines from a log file : for example the following logfile : SQL> SQL> SET PAGESIZE 0 SQL> SELECT 'Starting apply for patch 26925263/21857460 on ' || 2 SYSTIMESTAMP FROM dual; Starting apply for patch 26925263/21857460 on 28-JUN-18 07.46.34.893957 PM -05:00 1 row selected. SQL> SET PAGESIZE 10 SQL> SQL> BEGIN 2 dbms_sqlpatch.patch_initialize(p_patch_id => 26925263, 3 p_patch_uid => 21857460, 4 p_flags => '&flags', 5 p_description => '&description', 6 p_action => 'APPLY', 7 p_logfile => '&full_logfile', 8 p_bundle_series => '&bundle_series'); 9 END; 10 / BEGIN * ERROR at line 1: ORA-20001: set_patch_metadata not called ORA-06512: at "SYS.DBMS_SQLPATCH", line 647 ORA-06512: at line 2 SQL> SQL> COLUMN install_file NEW_VALUE sql_script SQL> SQL> SQL> ALTER SESSION SET CURRENT_SCHEMA = SYS; Session altered. So here I want to open the log file and find the error and print all the lines from starting till the error : ORA-20001: set_patch_metadata not called f4 = open (r"file1.log", 'r') string2=f4.readlines() for i in range(len(string2)): position=i lastposition =position+1 while True: if re.search('ORA-20001: set_patch_metadata not called',string2[lastposition]): break elif lastposition==len(string2)-1: break else: lastposition += 1 errorcheck=string2[position:lastposition] print errorcheck It gives me error : IOPub data rate exceeded. The notebook server will temporarily stop sending output to the client in order to avoid crashing it. To change this limit, set the config variable `--NotebookApp.iopub_data_rate_limit`. Current values: NotebookApp.iopub_data_rate_limit=1000000.0 (bytes/sec) NotebookApp.rate_limit_window=3.0 (secs) the first for loop will search for the error and provide me the position of the error . I think the logic I am using is correct but need some expert comment to make it work also some guidance on how to print all the lines until the error is received . Please advice, -- Asad Hasan +91 9582111698

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Error Python version 3.6 does not support this syntax.
by srinivasan Nov. 30, 2018

Nov. 30, 2018

Dear Python Experts, As still I am newbie and learning python, I am trying to reuse the Bluetoothctl wrapper in Python from the link ( https://gist.github.com/egorf/66d88056a9d703928f93) I am using python3.6 version, In pycharm editor on the bold highlighted code snippets I see the error message "Python version 3.6 does not support this syntax.", Could you please how help me how the below highlighted lines of code can be can be ported to python3.6 version? * except BluetoothctlError, e:* * print(e)* * return None* Full Code snippet: ============== import time import pexpect import subprocess import sys class BluetoothctlError(Exception): """This exception is raised, when bluetoothctl fails to start.""" pass class Bluetoothctl: """A wrapper for bluetoothctl utility.""" def __init__(self): out = subprocess.check_output("rfkill unblock bluetooth", shell = True) self.child = pexpect.spawn("bluetoothctl", echo = False) def get_output(self, command, pause = 0): """Run a command in bluetoothctl prompt, return output as a list of lines.""" self.child.send(command + "\n") time.sleep(pause) start_failed = self.child.expect(["bluetooth", pexpect.EOF]) if start_failed: raise BluetoothctlError("Bluetoothctl failed after running " + command) return self.child.before.split("\r\n") def start_scan(self): """Start bluetooth scanning process.""" try: out = self.get_output("scan on") * except BluetoothctlError, e: print(e) return None* def make_discoverable(self): """Make device discoverable.""" try: out = self.get_output("discoverable on") * except BluetoothctlError, e: print(e) return None* def parse_device_info(self, info_string): """Parse a string corresponding to a device.""" device = {} block_list = ["[\x1b[0;", "removed"] string_valid = not any(keyword in info_string for keyword in block_list) if string_valid: try: device_position = info_string.index("Device") except ValueError: pass else: if device_position > -1: attribute_list = info_string[device_position:].split(" ", 2) device = { "mac_address": attribute_list[1], "name": attribute_list[2] } return device def get_available_devices(self): """Return a list of tuples of paired and discoverable devices.""" try: out = self.get_output("devices") * except BluetoothctlError, e: print(e) return None* else: available_devices = [] for line in out: device = self.parse_device_info(line) if device: available_devices.append(device) return available_devices def get_paired_devices(self): """Return a list of tuples of paired devices.""" try: out = self.get_output("paired-devices") * except BluetoothctlError, e: print(e) return None* else: paired_devices = [] for line in out: device = self.parse_device_info(line) if device: paired_devices.append(device) return paired_devices def get_discoverable_devices(self): """Filter paired devices out of available.""" available = self.get_available_devices() paired = self.get_paired_devices() return [d for d in available if d not in paired] def get_device_info(self, mac_address): """Get device info by mac address.""" try: out = self.get_output("info " + mac_address) * except BluetoothctlError, e: print(e) return None* else: return out def pair(self, mac_address): """Try to pair with a device by mac address.""" try: out = self.get_output("pair " + mac_address, 4) * except BluetoothctlError, e: print(e) return None* else: res = self.child.expect(["Failed to pair", "Pairing successful", pexpect.EOF]) success = True if res == 1 else False return success def remove(self, mac_address): """Remove paired device by mac address, return success of the operation.""" try: out = self.get_output("remove " + mac_address, 3) * except BluetoothctlError, e: print(e) return None* else: res = self.child.expect(["not available", "Device has been removed", pexpect.EOF]) success = True if res == 1 else False return success def connect(self, mac_address): """Try to connect to a device by mac address.""" try: out = self.get_output("connect " + mac_address, 2) * except BluetoothctlError, e: print(e) return None* else: res = self.child.expect(["Failed to connect", "Connection successful", pexpect.EOF]) success = True if res == 1 else False return success def disconnect(self, mac_address): """Try to disconnect to a device by mac address.""" try: out = self.get_output("disconnect " + mac_address, 2) * except BluetoothctlError, e: print(e) return None* else: res = self.child.expect(["Failed to disconnect", "Successful disconnected", pexpect.EOF]) success = True if res == 1 else False return success kindly do the needful, Many Thanks in advance,

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Trouble about creating database
by Muhammed Esen Nov. 29, 2018

Nov. 29, 2018

I can't create a database even tough I imported pymysql. I have Xammp on my Mac and I already started MySQL Database. What could be the problem? Thanks have a nice day.

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Re: [Tutor] writer overloaded
by Avi Gross Nov. 29, 2018

Nov. 29, 2018

Steve, I read your reply in which you address selected parts of my earlier message. I conclude I am either not writing as clearly as I hoped or your interpretation is sometimes not what I intend. And, I can also be wrong or incomplete! I won't reply in-line or comment much on what I accept or agree with. Your comment about a good reason for the change in error handling makes perfect sense. But it relates to what I said in a way. The sequence of values allowed is a tuple of any length but it there are two or more, it gets ambiguous whether the last one is a special one used as a variable name you assign the particular error to or yet another kind of error. Adding "as" changes the syntax to "tuple [as name]" which is unambiguous and also allows "name" to be a reuse of a name that might otherwise happen to be an Error that wouldn't happen in this context but does exist. My focus is on what I said about Mathematics. I have very specific approaches to what I mean by mathematics and it is a very broad field with many seemingly different kinds which may overlap on careful study but which can each have their own use of concepts and symbols. Yes, mathematics as in routine algebra and geometry has a fairly serious focus on equality, often tautology, especially in higher and more abstract forms as when making proofs step by step. There are related concepts like congruence where you might not use an equals sign or inequalities where you use variants like <=. When a mathematician wants to SET a variable to a value, they often say things like: Given: Y = 5 Or Let Y = 5 when X = 10 be the sides of a rectangle then the Area = X*Y Or other phrases like "assuming" where the meaning remains that the combined expression on either side of the equals sign evaluates to the same thing. (X+1)*(X-1) = X^2 - 1 literally tells you there are at least two ways to say something that are the same no matter what X is. tan(x) =- sin(x)/cos(x) Is the same idea but maybe not exactly for the reason you mention that you can calculate it that way. The definition of the sin(x) is based on a Euclidean right triangle where you have two other angles that are not 90 degrees (or pi/2 radians) and you pick one of those angles and call it x. For each such angle, there are two sides of the triangle that delineate the angle with one being called the "hypotenuse" (the longest one) and one being called "adjacent". I know this is very basic geometry trending into trigonometry but trying to be very clear here. The third side sort of "opposite" the angle is called that. Now the definition of sin(x) is the ratio of opposite/hypotenuse. When the angle is 0, there is no triangle, As it opens up wider toward 90 degrees, the ratio climbs from 0 to as high as 1, asymptotically. At the same time as the opposite side lengthens, the adjacent side gets smaller and the cos(x) is defined as adjacent/hypotenuse and moves from 1 down towards 0. If you define the tangent as opposite/adjacent, it can go towards plus and minus infinity but someone can show that the ratio of: sin(x)/cos(x) = (opposite/hypotenuse)/(adjacent/hypotenuse) the latter part can be converted from a division to a multiplication by using the reciprocal and other allowed mathematical conversions in the context of this algebra. = opposite/hypotenuse * hypotenuse/ adjacent = (opposite*hypotenuse)/(adjacent*hypotenuse) = (opposite/adjacent)*(hypotenuse/hypotenuse) = (opposite/adjacent) + 1 = opposite/adjacent = tan(x) There are many details in the proof above, or similar ones like when you are allowed to use associative or commutative laws because the real numbers over the operators of addition and multiplication form a ring which has been proven to allow the steps above but you get the idea. So, yes, one of many ways to calculate a tangent is to use the sine or cosine for a particular known angle but there are deeper ways such as evaluating enough terms of the right infinite series. I repeat. In most of mathematics "=" means they are equal or assumed to be equal with any assignment taking place sort of off stage. Feel free to disagree on what may be a mostly philosophical point but that is why many places in mathematics do not feel a need for a second operator. I am trying to keep this as "text" since the mailing list does not tolerate rich formats so it is harder to illustrate. But there are mathematical operators that make provisional assignments possible. The upside-down A can be used to mean "for all x" and the backwards E can mean "there exists an x" and there are ways to specify conditions and intended results and so on. So yes, Mathematics as a whole can say the kind of things you want. But generally the way you might say to do something to x and store the results is to introduce a different variable such as x' and then x''. Programming languages definitely need a way to rebind but you could write perfectly usable code like this: x = 5 temp = x x = temp^2 +2*temp +1 The above would still rebind x but would not show it having two different values at the same time, seemingly on the same line. But, as long as you understand precedence rule or use lots of parantheses, it is clear that everything to the left of the equals sort of happens first. Mind you, the right side can be complex and take a while to evaluate to an object such as: a.b()["key1"]["key2"][5] = a a might have a method called b which returns a dictionary whose entry for key1 is yet another dictionary whose result is a list whose sixth entry you want to set to point back to what a is or is it was? And as noted, in languages that support pre as well as post autoincrement operators, you might need multiple temp variables to do the equivalent of: while (*p++ == *++q) ; bad example as python hides pointers so how about something else python does not support 😊 x++ = --y Your point about peano arithmetic is indeed valid. There are many places where you can indeed have something that looks nonsensical still have valid solutions and even an infinite number of such solutions. x = (x + 5) mod N is sort of what you meant. And I don't see what that disproves much as it involves a new operator. Both standard mathematics and python would see that as valid. In your example, both x are the same. I mean the same each time you try a new solution, not always the same. Similarly, asking what angle has a particular sine value has an infinite number of answers as adding or subtracting any integral multiple of 360 degrees (or 2*pi radians) gives you the same result. That is partially because this is in a sense peano arithmetic mod 360. 😊 The mathematical definition of "=" in such cases is not really different. Any of the valid answers is equal to the result, although for some purposes, you want the lowest positive answer meaning between 0 and 360. And the result is in a context. Complex numbers are hard to visualize but they are often the solutions to quadratic or higher-order equations. Without that context, you say there are no solutions, or at least two fewer if they are complex conjugates. If the two roots of an equation are: x=5 OR x=6 There is no contradiction. Two cases are being examined. X is at times one and at times another, but never both at the same time. You are quibbling about what I mean about the python "is" statement. Yes, it is an identity test but I repeat. I stand by my statement. It is about pointers or memory locations being compared for absolute equality. Does the first variable or expression refer to or point to the same location in memory as the second. I gave examples of how python when trying to use less memory interferes by making two different objects share the same area of memory in some cases when they are deemed immutable. It may not matter then but if your code depends on them not being seen as equal, you may have to do weird things like making them into objects that may internally point to the same string or number but then ask if object A is object B and then they are not. When discussing whether the absolute values of two things are the same, you are saying something different than whether the two things are the same. Similar the length/magnitude of two vectors (in the mathematical sense of the square root of the sum of the squares denoting a distance from a multi-dimensional origin in N-space) can be equal even if the two vectors share no components. The address of an object is another such function and "is" generally seems to be testing the current address, sort of. There may well be exceptions. I stand by my overall point. There are MANY ways people compare things for equality and some have subtle nuances. If this were something like C where pointers are commonly used then p == q and *p == *q Have related but different meanings. If p points to the first 'l' (el) in "hello world" and so does q then both statements are true. If you move q to point to the second or third letter 'l' while leaving p in place, then only *p == *q is true. If q points to any non-'l' character in the same string or anywhere else where the byte pointed to (pre-unicode) does not have the same eight bits in the same order, then both of the above are false. In python they hide pointers but they are probably there in many of the underlying implementations, albeit some use other techniques with name spaces and dictionaries. And who knows what else. So in many cases, "p is q" means identity in a pointer way or memory location way. But clearly there can be exceptions such as evaluating p searches in one namespace and evaluating q in another, so they may not be deemed to be identical and actually aren't. You may have also missed my last point. Short circuit evaluation is old. That is not what I consider unusual in python. What I found to be different (and it is possible other languages had or now have it) is that it does not bother jumping into a mathematical Boolean mode. That is deferred indefinitely as not important. The result of quite a few operations is neither True nor False but just about ANYTHING. To explicitly make it force a Boolean result you need to call bool(A or B and not C) which otherwise simply return A or B or C, whatever they are and the truthiness of it is determined by either it having a length of zero, or being numerically 0 or whatever the __nonzero__ (pre 3.X) or __bool__ methods for that object return, perhaps recursively if they return another object. It turns out, mathematically, to be valid and work in an amazing number of contexts and even where it might not, you can probably use the __bool__ method to make something that works. Sorry to be this long. I could have summed up my entire previous message in a few sentences but then you might have said I was too broad and showed no reasons or examples. As I said, life is short. -----Original Message----- From: Tutor <tutor-bounces+avigross=verizon.net(a)python.org> On Behalf Of Steven D'Aprano Sent: Wednesday, November 28, 2018 6:15 PM To: tutor(a)python.org Subject: Re: [Tutor] user overloaded On Wed, Nov 28, 2018 at 12:56:32PM -0500, Avi Gross wrote: > try ... > except ValueError, e: > ... > > You now need to do this format: > > try ... > except ValueError as e: > ... > > Why the change? Because the first form was a trap and a bug magnet: try: block except TypeError, ValueError, KeyError: print "surprise! this will not catch key errors!" [...] > Back to your topic. "=" as a symbol has been used over the years in > many ways. In normal mathematics, it is mostly seen as a comparison > operator as in tan(x) = sin(x)/cos(x) That's not a comparison, that's an identity. The tangent function is (or at least can be) defined as sine divided by cosine. > Many programming languages used it instead to sort of mean MAKE EQUALS. That's usually called "assignment" or "binding". > x = x + 5 > > is an example where it makes no mathematical sense. How can a number be > equal to itself when 5 is added? There is no solution there in the normal real number system, but there are forms of arithmetic where it does, e.g. "clock arithmetic". If your clock only has values 0 through 4, then adding 5 to any x will give you x back again. Although normally we write "congruent" rather than "equals" for clock arithmetic: x ≡ x + 5 (mod 5) In any case, just because there is no solution doesn't mean that the question makes no sense: x = x*5 makes perfect sense and has solution 0. Likewise: x = x**2 + k makes perfect sense too, and can have zero, one or two solutions depending on the value of k. > The meaning clearly is something like x > gets assigned a value by evaluating the current value of x and adding 5 to > that. When done, the old value of x is gone and it has a new value. > > For this reason, some languages like PASCAL used := so the above became > > x := x + 5 > > Yes, same symbol we are discussing and NOT what Python intends. Actually, it is precisely what Python intends, except for two minor differences: 1. in Pascal, := assignment is a statement, not an expression, while in Python it will be an expresion; 2. Python will discourage people from using the := expression form as a de facto statement, where = will do. [...] > And there are forms of equality that some languages need to distinguish. > What does it mean if two (unordered) sets are equal? Must they be two > reference to the same set or is it shorthand for them being strict subsets > of each other in both directions? You can come up with additional > abstractions like whether a list is a shallow or deep copy of another and > how it effects your ideas about them being equal. You may want a way to say > things are approximately equal. In math, something like ~= is used. Python > has an operator called "is" that can be a variant on equality. The "is" operator is in no way, shape or form an equality test. It is an *identity* test, which returns True if and only if the two operands are the same object. Equality does not imply identity: py> x = [] py> (x == []) and (x is not []) True nor does identity imply equality: py> NAN = float(NAN) py> (NAN is NAN) and (NAN != NAN) True Anyone using "is" when then test for equality is programming in a state of sin. > Consider the concept of OR. Some languages spell it out when used in a > Boolean context. > > a or b > > But python does something new with this. In some contexts the above > effectively becomes an IF. This is called a "short-circuiting boolean operator", and it was old, old old when Python started using it 25+ years ago. I believe Lisp was the first to introduce them, in the 1950s. https://en.wikipedia.org/wiki/Short-circuit_evaluation -- Steve _______________________________________________ Tutor maillist - Tutor(a)python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor

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online interactive Py resource?
by Mats Wichmann Nov. 29, 2018

Nov. 29, 2018

I'm going to end up tutoring someone (family member) learning Python in the new year. Look forward to it. It will be a remote arrangement. Haven't trained anyone in the whole picture (as opposed to bits and bobs of possibly helpful advice on demand, like here) since I wrote and delivered a training course for a commercial training company, the last instance of which was pre-historic in tech terms (2002 in calendar terms). That those were very much hands-on, in-person. It occurs to me it would be cool to work interactively in a distributed Internet editing environment that could run Python code. Are there such? There are distributed editing environments, like Etherpad, Gobby, (cough) Google Docs. There are online interactive Python environments, such as the ones from the nice folks at PythonAnywhere. is there something that combines the two? I'm not completely sure PythonAnywhere doesn't have that capability... but I'm looking for some wisdom and experiences (and maybe best practices) from the other tutors around here. cheers, -- mats

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user overloaded
by Avi Gross Nov. 28, 2018

Nov. 28, 2018

OOPS, Sorry Steve. Yes, I am on multiple mailing lists and the tutor list by default replies to the sender and you need to edit it to reply to the group. I will be careful. Your other point is broader. There are lots of operations (not just ion python) that are easy to misuse. Sometimes the same symbol is used in multiple ways, such as how parentheses can mean a tuple even when you wanted to use them for ordering. There was a recent discussion about how the try statement changed after 2.x so instead of the part that used to say: try ... except ValueError, e: ... You now need to do this format: try ... except ValueError as e: ... Why the change? I have not looked to see, but note the earlier version is sort of a tuple without parentheses which Python allows in many contexts. As the keyword "as" is now used in other places like import statements so why not make things more consistent. But to others who just see a CHANGE and wonder why, many things seem mysterious. Yes, read the manual but that is not something new users seem to want to do, let alone know how to find what to read and slog through lots of pages. Back to your topic. "=" as a symbol has been used over the years in many ways. In normal mathematics, it is mostly seen as a comparison operator as in tan(x) = sin(x)/cos(x) Many programming languages used it instead to sort of mean MAKE EQUALS. x = x + 5 is an example where it makes no mathematical sense. How can a number be equal to itself when 5 is added? The meaning clearly is something like x gets assigned a value by evaluating the current value of x and adding 5 to that. When done, the old value of x is gone and it has a new value. For this reason, some languages like PASCAL used := so the above became x := x + 5 Yes, same symbol we are discussing and NOT what Python intends. Languages like R also decided to not use a naked "=" sign and encouraged arrow notation like: x <- x + 5 and even x + x -> x And yes, for global variables there are --> and <-- and worse. But they still accept a naked "=" as well and use that alone in function definitions and calls for setting values by name. Yes, it can confuse people when an assignment is "<-" and a comparison is "<=" . And there are forms of equality that some languages need to distinguish. What does it mean if two (unordered) sets are equal? Must they be two reference to the same set or is it shorthand for them being strict subsets of each other in both directions? You can come up with additional abstractions like whether a list is a shallow or deep copy of another and how it effects your ideas about them being equal. You may want a way to say things are approximately equal. In math, something like ~= is used. Python has an operator called "is" that can be a variant on equality. But it can be subtle when python reuses an immutable object and they seem to be the same even when you try to make them different: >>> a = "supercalifragilisticexpialidocious" * 20 >>> b = "supercalifragilisticexpialidocious" * 10 + "supercalifragilisticexpialidocious" * 10 >>> a == b True >>> a is b True Weird. Try that with mutables: >>> a = [ 'hello' ] >>> b = [ 'hello' ] >>> a == b True >>> a is b False For people new to computer science or a particular programming language, there can be too much and users get overloaded by all the overloading. Adding yet another variant to have a side-effect within an expression using ":=" will extend some of the power of python but at a cost for some, and especially new users. Consider the concept of OR. Some languages spell it out when used in a Boolean context. a or b But python does something new with this. In some contexts the above effectively becomes an IF. >>> a = 5 >>> b = 10 >>> x = a or b >>> x 5 >>> a = 0 >>> x = a or b >>> x 10 So because python has a creative definition of what is True and What is False, the above is sort of this code: if ( a EVALUATES TO TRUE) x = a else x = WHATEVER b EVALUATES to Since I chose integers, a is true whenever it is not precisely zero. For lists and many other objects, they are True if not empty. That is not the same meaning of "or" that many languages use. Add other meanings like a "bitwise or", "exclusive or" or the union of two sets and you can see people get confused especially when the same or similar symbols are used. Many languages have a symbol like "|" and another like "||" for example. Arguably python allows users to invent their own overloading in classes so that using some version of "or" between two objects means whatever you want it to mean. I recently (for fun) created a sort of auto-increment (as in the C/C++ symbol ++) so that if you had a counter called x, then adding ANYTHING to x using the += notation just incremented it by 1. X = odd_object(6) # X initialized to 6 X += 1 # X is now 7 X += 12 # X is now 8 as the __iadd__ method ignores the 12 and adds just 1 The above abomination does not replace ++ as an autoincrement but can give you some functionality but many users will have no idea what just happened. I note it can be worse in languages that allow you to create your own operators. I often use %>% in R for example and can make my own by placing almost any nonsense between percent signs and binding it to a function. As I understand it, back to Python, if adding a := to be used in expressions does not break any existing code, it may not be so bad. I mean nobody is supposed to be using that symbol now, so what does it hurt? Ah, but wait. If I had the previously discussed notion of checking whether the string I wanted to dynamically evaluate contained possibly dangerous code, would it be looking for a ":=" ? Heck, if I am parsing something and looking for a colon that indicates the beginning of a BODY, might it get confused by finding this colon in a wrong context? I conclude by saying life is complicated and then you die. I mean there are many places where similar but not identical things happen in languages as well as life. -----Original Message----- From: Tutor <tutor-bounces+avigross=verizon.net(a)python.org> On Behalf Of Steven D'Aprano Sent: Wednesday, November 28, 2018 4:13 AM To: tutor(a)python.org Subject: Re: [Tutor] PEP 572 -- Assignment Expressions On Tue, Nov 27, 2018 at 08:26:04PM -0500, Avi Gross wrote: > Ivo, You've replied to the wrong mailing list. Ivo wrote to Python-List, not Tutor. But now that you've raised the issue... [Ivo asked] > Why: > if (match := pattern.search(data)) is not None: > # Do something with match > > What is wrong with: > if match = pattern.search(data) is not None: > # Do something with match > > Assignment expression or assignment statement, it's an assignment, right? > It is very clear to everyone that it's an assignment! Can't it all > just be a "="? It seems that Ivo didn't read the PEP because that is discussed: https://www.python.org/dev/peps/pep-0572/#id32 Despite Ivo's statement that it is "clear to everyone", the problem is that whenever you see an assignment x=y in an assignment, there's no obvious way of telling whether they meant assignment or it was a mistyped equality test x==y. So it is not clear at all. In C, this confusion between = and == is a frequent source of serious bugs, including one attempt at inserting a back-door into the Linux kernel: https://freedom-to-tinker.com/2013/10/09/the-linux-backdoor-attempt-of-2003/ https://lwn.net/Articles/57135/ Requiring the colon x:=y makes it less likely to mess up the assignment or comparison. -- Steve _______________________________________________ Tutor maillist - Tutor(a)python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor

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