def format_meter(n, total, elapsed, ncols=None, prefix='', ascii=False, unit='it',
                     unit_scale=False, rate=None, bar_format=None, postfix=None,
                     unit_divisor=1000, initial=0, colour=None, **extra_kwargs):
        """
        Return a string-based progress bar given some parameters

        Parameters
        ----------
        n  : int or float
            Number of finished iterations.
        total  : int or float
            The expected total number of iterations. If meaningless (None),
            only basic progress statistics are displayed (no ETA).
        elapsed  : float
            Number of seconds passed since start.
        ncols  : int, optional
            The width of the entire output message. If specified,
            dynamically resizes `{bar}` to stay within this bound
            [default: None]. If `0`, will not print any bar (only stats).
            The fallback is `{bar:10}`.
        prefix  : str, optional
            Prefix message (included in total width) [default: ''].
            Use as {desc} in bar_format string.
        ascii  : bool, optional or str, optional
            If not set, use unicode (smooth blocks) to fill the meter
            [default: False]. The fallback is to use ASCII characters
            " 123456789#".
        unit  : str, optional
            The iteration unit [default: 'it'].
        unit_scale  : bool or int or float, optional
            If 1 or True, the number of iterations will be printed with an
            appropriate SI metric prefix (k = 10^3, M = 10^6, etc.)
            [default: False]. If any other non-zero number, will scale
            `total` and `n`.
        rate  : float, optional
            Manual override for iteration rate.
            If [default: None], uses n/elapsed.
        bar_format  : str, optional
            Specify a custom bar string formatting. May impact performance.
            [default: '{l_bar}{bar}{r_bar}'], where
            l_bar='{desc}: {percentage:3.0f}%|' and
            r_bar='| {n_fmt}/{total_fmt} [{elapsed}<{remaining}, '
              '{rate_fmt}{postfix}]'
            Possible vars: l_bar, bar, r_bar, n, n_fmt, total, total_fmt,
              percentage, elapsed, elapsed_s, ncols, nrows, desc, unit,
              rate, rate_fmt, rate_noinv, rate_noinv_fmt,
              rate_inv, rate_inv_fmt, postfix, unit_divisor,
              remaining, remaining_s, eta.
            Note that a trailing ": " is automatically removed after {desc}
            if the latter is empty.
        postfix  : *, optional
            Similar to `prefix`, but placed at the end
            (e.g. for additional stats).
            Note: postfix is usually a string (not a dict) for this method,
            and will if possible be set to postfix = ', ' + postfix.
            However other types are supported (#382).
        unit_divisor  : float, optional
            [default: 1000], ignored unless `unit_scale` is True.
        initial  : int or float, optional
            The initial counter value [default: 0].
        colour  : str, optional
            Bar colour (e.g. 'green', '#00ff00').

        Returns
        -------
        out  : Formatted meter and stats, ready to display.
        """

        # sanity check: total
        if total and n >= (total + 0.5):  # allow float imprecision (#849)
            total = None

        # apply custom scale if necessary
        if unit_scale and unit_scale not in (True, 1):
            if total:
                total *= unit_scale
            n *= unit_scale
            if rate:
                rate *= unit_scale  # by default rate = self.avg_dn / self.avg_dt
            unit_scale = False

        elapsed_str = tqdm.format_interval(elapsed)

        # if unspecified, attempt to use rate = average speed
        # (we allow manual override since predicting time is an arcane art)
        if rate is None and elapsed:
            rate = (n - initial) / elapsed
        inv_rate = 1 / rate if rate else None
        format_sizeof = tqdm.format_sizeof
        rate_noinv_fmt = ((format_sizeof(rate) if unit_scale else
                           '{0:5.2f}'.format(rate)) if rate else '?') + unit + '/s'
        rate_inv_fmt = (
            (format_sizeof(inv_rate) if unit_scale else '{0:5.2f}'.format(inv_rate))
            if inv_rate else '?') + 's/' + unit
        rate_fmt = rate_inv_fmt if inv_rate and inv_rate > 1 else rate_noinv_fmt

        if unit_scale:
            n_fmt = format_sizeof(n, divisor=unit_divisor)
            total_fmt = format_sizeof(total, divisor=unit_divisor) if total is not None else '?'
        else:
            n_fmt = str(n)
            total_fmt = str(total) if total is not None else '?'

        try:
            postfix = ', ' + postfix if postfix else ''
        except TypeError:
            pass

        remaining = (total - n) / rate if rate and total else 0
        remaining_str = tqdm.format_interval(remaining) if rate else '?'
        try:
            eta_dt = (datetime.now() + timedelta(seconds=remaining)
                      if rate and total else datetime.utcfromtimestamp(0))
        except OverflowError:
            eta_dt = datetime.max

        # format the stats displayed to the left and right sides of the bar
        if prefix:
            # old prefix setup work around
            bool_prefix_colon_already = (prefix[-2:] == ": ")
            l_bar = prefix if bool_prefix_colon_already else prefix + ": "
        else:
            l_bar = ''

        r_bar = f'| {n_fmt}/{total_fmt} [{elapsed_str}<{remaining_str}, {rate_fmt}{postfix}]'

        # Custom bar formatting
        # Populate a dict with all available progress indicators
        format_dict = {
            # slight extension of self.format_dict
            'n': n, 'n_fmt': n_fmt, 'total': total, 'total_fmt': total_fmt,
            'elapsed': elapsed_str, 'elapsed_s': elapsed,
            'ncols': ncols, 'desc': prefix or '', 'unit': unit,
            'rate': inv_rate if inv_rate and inv_rate > 1 else rate,
            'rate_fmt': rate_fmt, 'rate_noinv': rate,
            'rate_noinv_fmt': rate_noinv_fmt, 'rate_inv': inv_rate,
            'rate_inv_fmt': rate_inv_fmt,
            'postfix': postfix, 'unit_divisor': unit_divisor,
            'colour': colour,
            # plus more useful definitions
            'remaining': remaining_str, 'remaining_s': remaining,
            'l_bar': l_bar, 'r_bar': r_bar, 'eta': eta_dt,
            **extra_kwargs}

        # total is known: we can predict some stats
        if total:
            # fractional and percentage progress
            frac = n / total
            percentage = frac * 100

            l_bar += '{0:3.0f}%|'.format(percentage)

            if ncols == 0:
                return l_bar[:-1] + r_bar[1:]

            format_dict.update(l_bar=l_bar)
            if bar_format:
                format_dict.update(percentage=percentage)

                # auto-remove colon for empty `{desc}`
                if not prefix:
                    bar_format = bar_format.replace("{desc}: ", '')
            else:
                bar_format = "{l_bar}{bar}{r_bar}"

            full_bar = FormatReplace()
            nobar = bar_format.format(bar=full_bar, **format_dict)
            if not full_bar.format_called:
                return nobar  # no `{bar}`; nothing else to do

            # Formatting progress bar space available for bar's display
            full_bar = Bar(frac,
                           max(1, ncols - disp_len(nobar)) if ncols else 10,
                           charset=Bar.ASCII if ascii is True else ascii or Bar.UTF,
                           colour=colour)
            if not _is_ascii(full_bar.charset) and _is_ascii(bar_format):
                bar_format = str(bar_format)
            res = bar_format.format(bar=full_bar, **format_dict)
            return disp_trim(res, ncols) if ncols else res

        elif bar_format:
            # user-specified bar_format but no total
            l_bar += '|'
            format_dict.update(l_bar=l_bar, percentage=0)
            full_bar = FormatReplace()
            nobar = bar_format.format(bar=full_bar, **format_dict)
            if not full_bar.format_called:
                return nobar
            full_bar = Bar(0,
                           max(1, ncols - disp_len(nobar)) if ncols else 10,
                           charset=Bar.BLANK, colour=colour)
            res = bar_format.format(bar=full_bar, **format_dict)
            return disp_trim(res, ncols) if ncols else res
        else:
            # no total: no progressbar, ETA, just progress stats
            return (f'{(prefix + ": ") if prefix else ""}'
                    f'{n_fmt}{unit} [{elapsed_str}, {rate_fmt}{postfix}]')

        Return a string-based progress bar given some parameters

        Parameters
        ----------
        n  : int or float
            Number of finished iterations.
        total  : int or float
            The expected total number of iterations. If meaningless (None),
            only basic progress statistics are displayed (no ETA).
        elapsed  : float
            Number of seconds passed since start.
        ncols  : int, optional
            The width of the entire output message. If specified,
            dynamically resizes `{bar}` to stay within this bound
            [default: None]. If `0`, will not print any bar (only stats).
            The fallback is `{bar:10}`.
        prefix  : str, optional
            Prefix message (included in total width) [default: ''].
            Use as {desc} in bar_format string.
        ascii  : bool, optional or str, optional
            If not set, use unicode (smooth blocks) to fill the meter
            [default: False]. The fallback is to use ASCII characters
            " 123456789#".
        unit  : str, optional
            The iteration unit [default: 'it'].
        unit_scale  : bool or int or float, optional
            If 1 or True, the number of iterations will be printed with an
            appropriate SI metric prefix (k = 10^3, M = 10^6, etc.)
            [default: False]. If any other non-zero number, will scale
            `total` and `n`.
        rate  : float, optional
            Manual override for iteration rate.
            If [default: None], uses n/elapsed.
        bar_format  : str, optional
            Specify a custom bar string formatting. May impact performance.
            [default: '{l_bar}{bar}{r_bar}'], where
            l_bar='{desc}: {percentage:3.0f}%|' and
            r_bar='| {n_fmt}/{total_fmt} [{elapsed}<{remaining}, '
              '{rate_fmt}{postfix}]'
            Possible vars: l_bar, bar, r_bar, n, n_fmt, total, total_fmt,
              percentage, elapsed, elapsed_s, ncols, nrows, desc, unit,
              rate, rate_fmt, rate_noinv, rate_noinv_fmt,
              rate_inv, rate_inv_fmt, postfix, unit_divisor,
              remaining, remaining_s, eta.
            Note that a trailing ": " is automatically removed after {desc}
            if the latter is empty.
        postfix  : *, optional
            Similar to `prefix`, but placed at the end
            (e.g. for additional stats).
            Note: postfix is usually a string (not a dict) for this method,
            and will if possible be set to postfix = ', ' + postfix.
            However other types are supported (#382).
        unit_divisor  : float, optional
            [default: 1000], ignored unless `unit_scale` is True.
        initial  : int or float, optional
            The initial counter value [default: 0].
        colour  : str, optional
            Bar colour (e.g. 'green', '#00ff00').

        Returns
        -------
        out  : Formatted meter and stats, ready to display.
        
lib/tqdm/std.py