There are several ways to present the output of a program; data can be printedin a human-readable form, or written to a file for future use. This chapter willdiscuss some of the possibilities.
C:UsersMy Namepython demofileseek.py o! Welcome to demofile.txt. Result Size: 497 x 420. 2 is the cursor location after seek operation. 2 is the current cursor location as reported by tell operation. The next three lines are the content of the file returned by read method. Notice the missing 2 characters which have been skipped due to seek operation. Files for seek, version 0.2.3; Filename, size File type Python version Upload date Hashes; Filename, size seek-0.2.3.tar.gz (4.0 kB) File type Source Python version None Upload date Feb 12, 2015 Hashes View. August 31, 2020 Jay Excel, Python Post Views: 2,440 Excel provides an excellent feature – Goal Seek, which allows back-solving an input value when given the end result. It is a handy tool, so today we'll learn how to replicate Goal Seek in Python.
7.1. Fancier Output Formatting¶
So far we've encountered two ways of writing values: expression statements andthe print() function. (A third way is using the write() methodof file objects; the standard output file can be referenced as sys.stdout.See the Library Reference for more information on this.)
Often you'll want more control over the formatting of your output than simplyprinting space-separated values. There are several ways to format output.
To use formatted string literals, begin a stringwith
forFbefore the opening quotation mark or triple quotation mark.Inside this string, you can write a Python expression between{and}characters that can refer to variables or literal values.The
str.format()method of strings requires more manualeffort. You'll still use{and}to mark where a variablewill be substituted and can provide detailed formatting directives,but you'll also need to provide the information to be formatted.Finally, you can do all the string handling yourself by using string slicing andconcatenation operations to create any layout you can imagine. Thestring type has some methods that perform useful operations for paddingstrings to a given column width.
When you don't need fancy output but just want a quick display of somevariables for debugging purposes, you can convert any value to a string withthe repr() or str() functions.
The str() function is meant to return representations of values which arefairly human-readable, while repr() is meant to generate representationswhich can be read by the interpreter (or will force a SyntaxError ifthere is no equivalent syntax). For objects which don't have a particularrepresentation for human consumption, str() will return the same value asrepr(). Many values, such as numbers or structures like lists anddictionaries, have the same representation using either function. Strings, inparticular, have two distinct representations.
Some examples:
The string module contains a Template class that offersyet another way to substitute values into strings, using placeholders like$x and replacing them with values from a dictionary, but offers much lesscontrol of the formatting.
7.1.1. Formatted String Literals¶
Formatted string literals (also called f-strings forshort) let you include the value of Python expressions inside a string byprefixing the string with f or F and writing expressions as{expression}.
An optional format specifier can follow the expression. This allows greatercontrol over how the value is formatted. The following example rounds pi tothree places after the decimal:
Passing an integer after the ':' will cause that field to be a minimumnumber of characters wide. This is useful for making columns line up.
Other modifiers can be used to convert the value before it is formatted.'!a' applies ascii(), '!s' applies str(), and '!r'applies repr():
For a reference on these format specifications, seethe reference guide for the Format Specification Mini-Language.
7.1.2. The String format() Method¶
Basic usage of the str.format() method looks like this:
The brackets and characters within them (called format fields) are replaced withthe objects passed into the str.format() method. A number in thebrackets can be used to refer to the position of the object passed into thestr.format() method.
If keyword arguments are used in the str.format() method, their valuesare referred to by using the name of the argument.
Positional and keyword arguments can be arbitrarily combined:
If you have a really long format string that you don't want to split up, itwould be nice if you could reference the variables to be formatted by nameinstead of by position. This can be done by simply passing the dict and usingsquare brackets '[]' to access the keys.
This could also be done by passing the table as keyword arguments with the ‘**'notation.
This is particularly useful in combination with the built-in functionvars(), which returns a dictionary containing all local variables.
As an example, the following lines produce a tidily-alignedset of columns giving integers and their squares and cubes:
For a complete overview of string formatting with str.format(), seeFormat String Syntax.
7.1.3. Manual String Formatting¶
Here's the same table of squares and cubes, formatted manually:
(Note that the one space between each column was added by theway print() works: it always adds spaces between its arguments.)
The str.rjust() method of string objects right-justifies a string in afield of a given width by padding it with spaces on the left. There aresimilar methods str.ljust() and str.center(). These methods donot write anything, they just return a new string. If the input string is toolong, they don't truncate it, but return it unchanged; this will mess up yourcolumn lay-out but that's usually better than the alternative, which would belying about a value. (If you really want truncation you can always add aslice operation, as in x.ljust(n)[:n].)
There is another method, str.zfill(), which pads a numeric string on theleft with zeros. It understands about plus and minus signs:
7.1.4. Old string formatting¶
Python Seek Eof
The % operator (modulo) can also be used for string formatting. Given 'string'%values, instances of % in string are replaced with zero or moreelements of values. This operation is commonly known as stringinterpolation. For example:
More information can be found in the printf-style String Formatting section.
7.2. Reading and Writing Files¶
open() returns a file object, and is most commonly used withtwo arguments: open(filename,mode).
The first argument is a string containing the filename. The second argument isanother string containing a few characters describing the way in which the filewill be used. mode can be 'r' when the file will only be read, 'w'for only writing (an existing file with the same name will be erased), and'a' opens the file for appending; any data written to the file isautomatically added to the end. 'r+' opens the file for both reading andwriting. The mode argument is optional; 'r' will be assumed if it'somitted.
Normally, files are opened in text mode, that means, you read and writestrings from and to the file, which are encoded in a specific encoding. Ifencoding is not specified, the default is platform dependent (seeopen()). 'b' appended to the mode opens the file inbinary mode: now the data is read and written in the form of bytesobjects. This mode should be used for all files that don't contain text.
In text mode, the default when reading is to convert platform-specific lineendings (n on Unix, rn on Windows) to just n. When writing intext mode, the default is to convert occurrences of n back toplatform-specific line endings. This behind-the-scenes modificationto file data is fine for text files, but will corrupt binary data like that inJPEG or EXE files. Be very careful to use binary mode whenreading and writing such files.
It is good practice to use the with keyword when dealingwith file objects. The advantage is that the file is properly closedafter its suite finishes, even if an exception is raised at somepoint. Using with is also much shorter than writingequivalent try-finally blocks:
If you're not using the with keyword, then you should callf.close() to close the file and immediately free up any systemresources used by it.
Warning
Calling f.write() without using the with keyword or callingf.close()might result in the argumentsof f.write() not being completely written to the disk, even if theprogram exits successfully.
VIOS-L2 configuration file in GNS3 Whenever I stop the appliance and run it back, all the switch's configuration is gone even after issuing the 'copy running-config startup-config or copy running-config nvram: and copy running-config flash:'. Cisco vios l2 - viosl2-adventerprisek9. Cisco Virl Iosvl2 Image Simulator And Furthermore; Download vIOS-L2 By making use of the vIOS-L2 image, you can make use of many changing features like as Etherchannel, DTP, HSRP, Port Safety, VLAN Routing, Switchport, 802.1Q, Trunk area Port, Entry Interface. VIRL has two image formats vmdk and qcow2. Qcow2 are almost ready to use, just need create proper image folder and load image. Vmdk file need covert to qcow2 format before load it in EVE. In the table above you can see that vios advanced enterprise 156-1.T contains vmdk in the filename. This means will need convert it to qcow2 file. I managed to download viosl2-adventerprisek9-m.03.2017.vmdk and 'show version' gives below output. Is this latest IOSvL2, which will be bundled to next release of VIRL? Looks like this image is same as the IOSvL2 available in VIRL-on-Packet. If so, are there any document and/or write-ups regarding features and limitations of this image? You can use advanced switching technologies with Cisco Switch vIOS-L2 IOS image on GNS3 network simulator program.
After a file object is closed, either by a with statementor by calling f.close(), attempts to use the file object willautomatically fail.
7.2.1. Methods of File Objects¶
The rest of the examples in this section will assume that a file object calledf has already been created.
To read a file's contents, call f.read(size), which reads some quantity ofdata and returns it as a string (in text mode) or bytes object (in binary mode).size is an optional numeric argument. When size is omitted or negative, theentire contents of the file will be read and returned; it's your problem if thefile is twice as large as your machine's memory. Otherwise, at most sizecharacters (in text mode) or size bytes (in binary mode) are read and returned.If the end of the file has been reached, f.read() will return an emptystring (').
f.readline() reads a single line from the file; a newline character (n)is left at the end of the string, and is only omitted on the last line of thefile if the file doesn't end in a newline. This makes the return valueunambiguous; if f.readline() returns an empty string, the end of the filehas been reached, while a blank line is represented by 'n', a stringcontaining only a single newline.
For reading lines from a file, you can loop over the file object. This is memoryefficient, fast, and leads to simple code:
Passing an integer after the ':' will cause that field to be a minimumnumber of characters wide. This is useful for making columns line up.
Other modifiers can be used to convert the value before it is formatted.'!a' applies ascii(), '!s' applies str(), and '!r'applies repr():
For a reference on these format specifications, seethe reference guide for the Format Specification Mini-Language.
7.1.2. The String format() Method¶
Basic usage of the str.format() method looks like this:
The brackets and characters within them (called format fields) are replaced withthe objects passed into the str.format() method. A number in thebrackets can be used to refer to the position of the object passed into thestr.format() method.
If keyword arguments are used in the str.format() method, their valuesare referred to by using the name of the argument.
Positional and keyword arguments can be arbitrarily combined:
If you have a really long format string that you don't want to split up, itwould be nice if you could reference the variables to be formatted by nameinstead of by position. This can be done by simply passing the dict and usingsquare brackets '[]' to access the keys.
This could also be done by passing the table as keyword arguments with the ‘**'notation.
This is particularly useful in combination with the built-in functionvars(), which returns a dictionary containing all local variables.
As an example, the following lines produce a tidily-alignedset of columns giving integers and their squares and cubes:
For a complete overview of string formatting with str.format(), seeFormat String Syntax.
7.1.3. Manual String Formatting¶
Here's the same table of squares and cubes, formatted manually:
(Note that the one space between each column was added by theway print() works: it always adds spaces between its arguments.)
The str.rjust() method of string objects right-justifies a string in afield of a given width by padding it with spaces on the left. There aresimilar methods str.ljust() and str.center(). These methods donot write anything, they just return a new string. If the input string is toolong, they don't truncate it, but return it unchanged; this will mess up yourcolumn lay-out but that's usually better than the alternative, which would belying about a value. (If you really want truncation you can always add aslice operation, as in x.ljust(n)[:n].)
There is another method, str.zfill(), which pads a numeric string on theleft with zeros. It understands about plus and minus signs:
7.1.4. Old string formatting¶
Python Seek Eof
The % operator (modulo) can also be used for string formatting. Given 'string'%values, instances of % in string are replaced with zero or moreelements of values. This operation is commonly known as stringinterpolation. For example:
More information can be found in the printf-style String Formatting section.
7.2. Reading and Writing Files¶
open() returns a file object, and is most commonly used withtwo arguments: open(filename,mode).
The first argument is a string containing the filename. The second argument isanother string containing a few characters describing the way in which the filewill be used. mode can be 'r' when the file will only be read, 'w'for only writing (an existing file with the same name will be erased), and'a' opens the file for appending; any data written to the file isautomatically added to the end. 'r+' opens the file for both reading andwriting. The mode argument is optional; 'r' will be assumed if it'somitted.
Normally, files are opened in text mode, that means, you read and writestrings from and to the file, which are encoded in a specific encoding. Ifencoding is not specified, the default is platform dependent (seeopen()). 'b' appended to the mode opens the file inbinary mode: now the data is read and written in the form of bytesobjects. This mode should be used for all files that don't contain text.
In text mode, the default when reading is to convert platform-specific lineendings (n on Unix, rn on Windows) to just n. When writing intext mode, the default is to convert occurrences of n back toplatform-specific line endings. This behind-the-scenes modificationto file data is fine for text files, but will corrupt binary data like that inJPEG or EXE files. Be very careful to use binary mode whenreading and writing such files.
It is good practice to use the with keyword when dealingwith file objects. The advantage is that the file is properly closedafter its suite finishes, even if an exception is raised at somepoint. Using with is also much shorter than writingequivalent try-finally blocks:
If you're not using the with keyword, then you should callf.close() to close the file and immediately free up any systemresources used by it.
Warning
Calling f.write() without using the with keyword or callingf.close()might result in the argumentsof f.write() not being completely written to the disk, even if theprogram exits successfully.
VIOS-L2 configuration file in GNS3 Whenever I stop the appliance and run it back, all the switch's configuration is gone even after issuing the 'copy running-config startup-config or copy running-config nvram: and copy running-config flash:'. Cisco vios l2 - viosl2-adventerprisek9. Cisco Virl Iosvl2 Image Simulator And Furthermore; Download vIOS-L2 By making use of the vIOS-L2 image, you can make use of many changing features like as Etherchannel, DTP, HSRP, Port Safety, VLAN Routing, Switchport, 802.1Q, Trunk area Port, Entry Interface. VIRL has two image formats vmdk and qcow2. Qcow2 are almost ready to use, just need create proper image folder and load image. Vmdk file need covert to qcow2 format before load it in EVE. In the table above you can see that vios advanced enterprise 156-1.T contains vmdk in the filename. This means will need convert it to qcow2 file. I managed to download viosl2-adventerprisek9-m.03.2017.vmdk and 'show version' gives below output. Is this latest IOSvL2, which will be bundled to next release of VIRL? Looks like this image is same as the IOSvL2 available in VIRL-on-Packet. If so, are there any document and/or write-ups regarding features and limitations of this image? You can use advanced switching technologies with Cisco Switch vIOS-L2 IOS image on GNS3 network simulator program.
After a file object is closed, either by a with statementor by calling f.close(), attempts to use the file object willautomatically fail.
7.2.1. Methods of File Objects¶
The rest of the examples in this section will assume that a file object calledf has already been created.
To read a file's contents, call f.read(size), which reads some quantity ofdata and returns it as a string (in text mode) or bytes object (in binary mode).size is an optional numeric argument. When size is omitted or negative, theentire contents of the file will be read and returned; it's your problem if thefile is twice as large as your machine's memory. Otherwise, at most sizecharacters (in text mode) or size bytes (in binary mode) are read and returned.If the end of the file has been reached, f.read() will return an emptystring (').
f.readline() reads a single line from the file; a newline character (n)is left at the end of the string, and is only omitted on the last line of thefile if the file doesn't end in a newline. This makes the return valueunambiguous; if f.readline() returns an empty string, the end of the filehas been reached, while a blank line is represented by 'n', a stringcontaining only a single newline.
For reading lines from a file, you can loop over the file object. This is memoryefficient, fast, and leads to simple code:
If you want to read all the lines of a file in a list you can also uselist(f) or f.readlines().
f.write(string) writes the contents of string to the file, returningthe number of characters written.
Other types of objects need to be converted – either to a string (in text mode)or a bytes object (in binary mode) – before writing them:
f.tell() returns an integer giving the file object's current position in the filerepresented as number of bytes from the beginning of the file when in binary mode andan opaque number when in text mode.
To change the file object's position, use f.seek(offset,whence). The position is computedfrom adding offset to a reference point; the reference point is selected bythe whence argument. A whence value of 0 measures from the beginningof the file, 1 uses the current file position, and 2 uses the end of the file asthe reference point. whence can be omitted and defaults to 0, using thebeginning of the file as the reference point.
In text files (those opened without a b in the mode string), only seeksrelative to the beginning of the file are allowed (the exception being seekingto the very file end with seek(0,2)) and the only valid offset values arethose returned from the f.tell(), or zero. Any other offset value producesundefined behaviour.
File objects have some additional methods, such as isatty() andtruncate() which are less frequently used; consult the LibraryReference for a complete guide to file objects. Videoproc digiarty.
7.2.2. Saving structured data with json¶
Strings can easily be written to and read from a file. Numbers take a bit moreeffort, since the read() method only returns strings, which will have tobe passed to a function like int(), which takes a string like '123'and returns its numeric value 123. When you want to save more complex datatypes like nested lists and dictionaries, parsing and serializing by handbecomes complicated.
Rather than having users constantly writing and debugging code to savecomplicated data types to files, Python allows you to use the popular datainterchange format called JSON (JavaScript Object Notation). The standard module called json can take Pythondata hierarchies, and convert them to string representations; this process iscalled serializing. Reconstructing the data from the string representationis called deserializing. Between serializing and deserializing, thestring representing the object may have been stored in a file or data, orsent over a network connection to some distant machine.
Note
The JSON format is commonly used by modern applications to allow for dataexchange. Many programmers are already familiar with it, which makesit a good choice for interoperability.
If you have an object x, you can view its JSON string representation with asimple line of code:
Another variant of the dumps() function, called dump(),simply serializes the object to a text file. So if f is atext file object opened for writing, we can do this:
To decode the object again, if f is a text file object which hasbeen opened for reading:
This simple serialization technique can handle lists and dictionaries, butserializing arbitrary class instances in JSON requires a bit of extra effort.The reference for the json module contains an explanation of this.
Python Seek Backwards
See also
pickle - the pickle module
Python Seek To End Of File
Contrary to JSON, pickle is a protocol which allowsthe serialization of arbitrarily complex Python objects. As such, it isspecific to Python and cannot be used to communicate with applicationswritten in other languages. It is also insecure by default:deserializing pickle data coming from an untrusted source can executearbitrary code, if the data was crafted by a skilled attacker.
